British number two Kyle Edmund was beaten in straight sets by second seed Alexander Zverev in the first round of the China Open in Beijing.
Edmund, ranked 47th in the world, had the chance to serve for the second set but Zverev broke to force a tie-break.
The German went on to win 6-3 7-6 (7-3) and set up a second-round meeting with Italy's Fabio Fognini.
Earlier, world number one Rafael Nadal saved two match points as he came from a set down to beat Lucas Pouille.
Edmund, 22, was playing in his third match since he suffered a neck injuryduring the US Open in September.
He won the first five points of the match against Zverev, but an early break allowed the world number four to take the set, before a composed tie-break performance ensured victory.
Nadal, making his first appearance since last month's US Open win, was 4-6 down in the second set tie-break before winning four consecutive points.
The Spaniard, 31, then converted his only break point of the match in the final set to beat the French player 4-6 7-6 (8-6) 7-5.
"I am very, very happy to be through," said Nadal.
"He played well, I think, very aggressive. He's serving well. For me it was a little bit difficult at the beginning, then I started to play better.
"But still, I didn't have the control of the match for almost all the time."
Nadal, who won his 16th Grand Slam title in New York three weeks ago, goes on to play Russia's Karen Khachanov in the next round.
Elsewhere in Beijing, Australia's Nick Kyrgios beat Georgia's Nikoloz Basilashvili 6-1 6-2, Bulgaria's Grigor Dimitrov beat Bosnia's Damir Dzumhur 6-1 3-6 6-3 and Argentina's Juan Martin del Potro is also through after a 7-6 (7-4) 6-4 win against Pablo Cuevas of Uruguay.
American John Isner reached the second round with a 6-2 6-3 victory over Tunisia's Malek Jaziri, while Czech Tomas Berdych beat American Jared Donaldson 6-3 0-6 6-2.
New mother Serena Williams will lack some of her trademark intimidation factor when she returns to tennis, US great Chris Evert told AFP, warning that the 23-time Grand Slam-winner may find her comeback tough.
Williams, who gave birth to a baby girl in September, has said she's planning to defend her title at the Australian Open in January –- just four-and-a-half months after becoming a mother.
But Evert said standards in women's tennis had risen this year since Williams stepped off the tour, warning that the 36-year-old would be returning to a more competitive environment.
"I think that this whole year, the level has gone up because Serena hasn't been dominant and the other players have all felt that they had a shot at it, at the number one ranking," Evert said, speaking in her role as an ambassador for this month's WTA Finals in Singapore.
"I think that because of that challenge, they have improved in the physical and the mental part of it because they've all had a shot at it and they've pushed each other and that's why there have been so many different winners. So I think the level will have improved by the time Serena comes back."
Since Williams' last match –- January's Australian Open final against her sister Venus –- Jelena Ostapenko was a surprise winner of the French Open, Garbine Muguruza triumphed at Wimbledon and Sloane Stephens capped a brilliant return from injury to lift the US Open trophy.
Angelique Kerber, Karolina Pliskova and Muguruza, the current number one, have all held the top ranking this season, and Russia's Maria Sharapova is back on the scene after returning from a 15-month drug ban.
Rafael Nadal won his third US Open and 16th Grand Slam title with a one-sided victory over South Africa's Kevin Anderson in New York.
The world number one powered to a 6-3 6-3 6-4 victory in Sunday's final at Flushing Meadows.
The Spaniard, 31, has now won two Slams in the same year for the first time since 2013, having won the French Open in June.
"It's just unbelievable what happened this year," said Nadal.
"After a couple of years with some troubles, injuries, sometimes playing not good - since the beginning of the season it has been very, very emotional."
The US Open was the last Grand Slam where he was working alongside his uncle Toni, with Carlos Moya taking sole charge of coaching from 2018.
"I can't thank him enough for all the things he did for me," Nadal said of his uncle.
"Probably without him I would never be playing tennis and it's great I had somebody like him pushing me all the time."
The result means this year's major titles are shared between Nadal, at the French and US Opens, and Roger Federer, at the Australian Open and Wimbledon.
Federer remains at the top of the men's all-time Grand Slam list with 19 titles, three ahead of Nadal, who has now won 10 French titles, three US, two Wimbledon and one Australian.
Nadal went into the final with a 4-0 head-to-head record against Anderson, and he dominated once again.
The 2010 and 2013 champion lost just 15 points on serve in the entire match, did not face a break point and won all 16 points he finished at the net.
It was a ruthless demolition job by a man playing his 23rd Grand Slam final, up against an opponent in his first.
Anderson, 31, was the tallest player ever to feature in a Grand Slam final at 6ft 8in, and with a tournament-leading 114 aces was always going to rely on his serve to get a foothold in the match.
The signs were not good, therefore, when he had to fight through six deuces in his second and third service games as Nadal pressed hard.
Anderson clearly felt the need to play to his limits from the outset, attacking the net more than normal to try to keep the points short, but it brought mixed results at best.
The pressure told in game seven when the South African pulled a forehand into the tramlines facing a sixth break point.
Nadal would break again to seal the set with a wonderful backhand drop volley, and Anderson ended the set with just eight winners from 19 trips to the net.
Venus Williams reached her 23rd Grand Slam semi-final with a thrilling win over Petra Kvitova in a final-set tie-break.
The 37-year-old American won 6-3 3-6 7-6 (7-2) in front of a rapturous night-session crowd in New York.
"I was so fortunate to win that match," said Williams. "It came down to the wire and I'm hoping we have more matches like that."
She next faces Sloane Stephens, with all-American semi-finals possible.
Madison Keys and Coco Vandeweghe will try to join their compatriots in the last four when they play their quarter-finals on Wednesday.
"I would love the top four, five to be that [American] again," added Williams. "That would be huge."
Kvitova, 27, missed out on a first US Open semi-final, ending a remarkable run just nine months since she suffered a knive attack at her home, which left her requiring surgery to her playing hand.
"Everything she has gone through is unbelievable," said Williams.
"It's wonderful to see her back and playing. I was so excited to see her playing well, to be able to play her."
Given primetime billing in the opening night-session slot, the match began with errors from both women and ended in a gripping finale.
The pair had met five times before and each time they had needed three sets, twice a final-set tie-break, and once again there was nothing between them.
A weather warning in the second set prompted a delay of 10 minutes as the roof was brought across, and it intensified the atmosphere as the noise built from 23,000 spectators.
Williams gave up the first break of serve at 3-1 with three double faults, before reeling off five straight games as Kvitova racked up the errors.
A loose Williams game early in the second offered Kvitova the lifeline she needed, and she clung on through several edgy service games.
The Kvitova celebratory scream was now regularly reverberating around the stadium as every game seemed to throw up mini-crises - she saved five break points on her way to clinching the set.
Two brilliant cross-court backhands gave Kvitova an early break in the third but Williams recovered from 3-1 down, levelling when the Czech double-faulted.
The scrappy play of earlier was long forgotten as the pair traded service games, with Williams coming through a 10-minute game at 4-4 as Kvitova pressed hard.
It came down to a tie-break and a brilliant return at 1-1 was enough to give Williams the momentum in the decisive shootout.
Roared on by the New York crowd, the 2000 and 2001 champion powered 6-1 clear, enough breathing space to cope with a double fault on her first match point before converting her second.
"I have to say I felt every single one of you guys behind me, all 23,000," Williams told the crowd. "I didn't want to let you guys down."
Maria Sharapova's first Grand Slam tournament in 19 months ended with defeat by Anastasija Sevastova in the US Open fourth round.
Latvia's Sevastova, the 16th seed, won 5-7 6-4 6-2 to reach the quarter-finals in New York.
The 27-year-old will play Sloane Stephens in the quarter-finals, after the American beat Julia Goerges.
Sharapova, ranked 146th after returning from a doping ban in April, was given a wildcard into the main draw.
"It's been a really great ride in the last week," said the 30-year-old.
"I can take a lot from this week. It's great to get that major out of the way. It was an incredible opportunity. I'm very thankful for the opportunity.
"I did my best. I can be proud of that."
The 2006 champion had played just one match since May coming into Flushing Meadows, with injuries forcing her out of the grass-court season and US Open build-up.
She played superbly to upset world number two Simona Halep on the first day of the tournament, and then beat Timea Babos and Sofia Kenin on her way to the last 16.
Sharapova's presence in the main draw divided opinion from the moment her wildcard was announced, but she was unperturbed as she racked up three wins in a row for the first time since her comeback in April.
The Russian was warmly received by spectators and organisers were happy to draw on her star power, putting her on the main Arthur Ashe Stadium for all four of her matches.
That decision was described as "questionable" by fifth seed Caroline Wozniacki, but Sharapova simply responded that scheduling was not her responsibility.
Asked after Sunday's defeat if she felt as though she had a target on her back, Sharapova responded: "I feel like I'm really beyond that. I mean, there's no other way to explain it.
"I think there's only a way to show it on the court, because that's what really matters to me.
"I have so many things in my life that I've already been able to experience, but there's a desire to keep going for more, and to keep training and to keep living through these moments out on these courts.
"That's special and that's meaningful. As long as I have that desire, I'll be there. That's what's important to me."