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.
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.
Rafael Nadal's return to the top of the world rankings is "as impressive as any" of the 15-time Grand Slam winner's achievements, says men's tennis chief Chris Kermode.
The Spaniard, 31, replaced Britain's Andy Murray as world number one on Monday - the first time since July 2014 he has held top spot.
It comes despite dealing with a series of knee and wrist injuries since first holding top ranking in 2008.
"It is unprecedented," said Kermode.
"Rafa has been setting records throughout his remarkable career and this one is as impressive as any. It shows incredible dedication and longevity," the ATP president added.
Nadal's recent rise has been helped by injuries to rivals such as Murray, who is recovering from a hip problem.
A back injury for 19-time Grand Slam winner Roger Federer and an elbow problem for 12-time Slam winner Novak Djokovic have similarly helped him in his rise.
And Nadal, who previously spent 141 weeks as world number one, said: "Being number one after all the things that I have been going through the last couple of years is something unbelievable."
He later told fans on social media: "Today is a special day, going back to number one, I'm very happy."
Rafael Nadal extended his unbeaten run over Frenchman Richard Gasquet to 15 to reach the Cincinnati Open third round.
The Spaniard, 31, who has not lost a set to Gasquet since 2008, won 6-3 6-4.
Roger Federer's withdrawal from the tournament means on Monday Nadal will return to world number one for the first time since July 2014, taking over from Andy Murray who is also injured.
Meanwhile, Alexander Zverev's 10-match winning streak ended with defeat by unseeded American Francis Tiafoe.
Zverev, 20, has moved up to seventh in the world rankings following victory over Roger Federer in Montreal.
But he lost 6-4 3-6 4-6 to 19-year-old Tiafoe, the world number 87.
Zverev arrived in Cincinnati having won back-to-back titles in Washington and Montreal, and he will start among the favourites when the US Open gets under way on 28 August.
"I'm not a machine. I'm tired. There was no hiding it. There's a physical limit to all of us and I've reached that now," Zverev admitted.
"This match is already forgotten. But it's something that's normal. It happens to all of us."
Nadal, who won a record 10th French Open in June, will next face fellow Spaniard Albert Ramos-Vinolas.
"I work every day to play better. It's obvious that was an important start for me, a good one."
In the women's event, world number nine Venus Williams was knocked out in the second round by Australian qualifier Ashleigh Barty.
Barty, ranked 48 in the world, beat the Wimbledon finalist 6-3 2-6 6-2 to earn her first victory over a top-10 player.
Williams, 37, served up six double faults in an error-strewn performance.
Barty, 21, will play world number five Caroline Wozniacki in the third round.
"It's an honour to walk out on the the same court as Venus, to play against her and share the court with her," Barty said.
"She's a genuine champion of the game and has, I think, paved the way for us young players coming through so to play like I did, I'm really excited."
Dane Wozniacki, one of four women who could replace Karolina Pliskova as world number one if results go her way this week, beat Russia's Elena Vesnina 6-2 6-4 to progress.
But German Angelique Kerber, who could have moved up from third to first in the rankings, was beaten in a third-set tie-break by Russian Ekaterina Makarova, who converted her eighth match point to win 6-4 1-6 7-6 (13-11).
Simona Halep and Elina Svitolina, the final two women in contention for the number one spot, are yet to play their second round matches.
Elsewhere, American Madison Keys saw off Russia's Daria Kasatkina 6-2 6-1, to set up a match against Wimbledon champion and fourth seed Garbine Muguruza of Spain.
Rafael Nadal will be the new world number one from next Monday after Roger Federer withdrew from the Cincinnati Masters with a back injury.
The Swiss 19-time Grand Slam winner was the only player who could have denied Spain's Nadal taking the top spot in the rankings from Andy Murray.
Federer was injured in Sunday's Rogers Cup final defeat by Alexander Zverev.
Briton Murray pulled out of Cincinnati last week as he continues to recover from a hip problem.
Reigning French Open champion Nadal returns to the top of the rankings for the first time since July 2014.
The 31-year-old went out of the Rogers Cup in the last 16 against wildcard Denis Shapovalov.
Federer went all the way to the final before losing to 20-year-old German Zverev in Montreal - the Swiss' first tournament since winning Wimbledon for the eighth time in July.
"I am very sorry to pull out," said the world number three. "Cincinnati has some of the best fans in the world and I am sorry I will miss them.
"Unfortunately, I tweaked my back in Montreal and I need to rest this week."
The Cincinnati Master leads up to the the US Open, the final Grand Slam of the year, which begins on 28 August.
Federer, 36, joins fellow top 10 players Murray, Kei Nishikori and Marin Cilic in withdrawing, while 2016 US Open winner Stan Wawrinka and 12-time Grand Slam champion Novak Djokovic will miss the rest of the season.