Britain's Andy Murray says he may have to consider surgery after withdrawing from the Brisbane International because of his long-term hip injury.
The 30-year-old is down to 16th in the world, having not played since July.
Murray said on Instagram surgery was a "secondary option, something I may have to consider but let's hope not".
The Australian Open starts in Melbourne on 15 January and Murray added he would decide by the weekend whether to stay in Australia or fly home.
Murray had been given a bye to the last 16 in Brisbane and was due to play American Ryan Harrison on Thursday.
The former world number one said: "It's quite demoralising that when you get on the court it's not at the level you need it to be to compete at this level. It's really hurting inside."
In an earlier statement announcing he would not be playing in Brisbane, three-time Grand Slam champion Murray, who won the event in 2012 and 2013 said: "I'm very disappointed to be withdrawing.
"I came here with every intention of making a strong start to the year."
Murray has not played competitively since his defence of the Wimbledon singles title ended in a five-set defeat by Sam Querrey in July, when he was hampered by the hip injury.
He made an unsuccessful attempt to return at the US Open in August, pulling out two days before the start of the tournament.
His only public on-court appearances since then have been in exhibitions against Roger Federer in November, and on Friday against Roberto Bautista Agut in Abu Dhabi.
In a recent interview, Murray said he would be content to be playing at "30 in the world level" given his lengthy injury problems.
"I just want to enjoy playing again. I've really missed it the last six months or so," he said.
Serena Williams will return to tennis in Abu Dhabi next week, almost four months after giving birth.
The American, 36, will play world number seven Jelena Ostapenko in an exhibition match on 30 December during the Mubadala World Tennis Championship.
Williams, who has won an Open-era record 23 Grand Slams, said she was "delighted to be returning to the court".
She gave birth to daughter Alexis Olympia Ohanian in September.
Former world number one Williams has not played since winning the Australian Open in January.
Coach Patrick Mouratoglou said in November that no decision had been made over whether Williams would play in the season's first Grand Slam.
Australian Open director Craig Tilley has said Williams is "very likely" to defend her title at the 2018 tournament, which starts on 15 January.
Ranked 22nd in the world, she would not need a wildcard.
Rafael Nadal, Milos Raonic and Stan Wawrinka have pulled out of the Mubadala World Tennis Championship, which runs from 28-30 December.
Latvian Ostapenko, whose match against Williams will be the first between women to be played at a tournament first staged in 2009, said: "It is a huge honour to be part of that history."
Serena Williams has entered the Australian Open in January, tournament director Craig Tiley has said.
The 36-year-old won this year’s event in Melbourne while in the early weeks of pregnancy and gave birth to a daughter in September.
Tiley told the Herald Sun the 23-time Grand Slam winner is "very likely" to return to tennis in Melbourne.
"She's got her visa, she's entered, she's practicing," he said. "There's no question she'll be ready in our view."
Williams, who has won the Australian Open seven times, has posted on social media that she has returned to training.
Australia's Margaret Court, with 24, is the only player still ahead of Serena in terms of Grand Slam singles titles.
"She wants to break a record that is Margaret Court's," added Tiley. "It would be a pretty significant accomplishment for her to be able to do that."
Venus Williams beat Wimbledon champion Garbine Muguruza in straight sets to reach the last four at the WTA Finals, ending the Spaniard's hopes of finishing the year as world number one.
The American, 37, won 7-5 6-4 in Singapore to secure second place in the White Group behind Karolina Pliskova.
Czech Pliskova had already qualified, but ended the round-robin stage with a 6-3 6-1 loss to Jelena Ostapenko.
Williams will play Caroline Wozniacki or Caroline Garcia in the semis.
Denmark's Wozniacki has guaranteed a semi-final place with two wins, and will win the White Group with victory over French eighth seed Garcia on Friday.
World number one Simona Halep of Romania then takes on Ukraine's fourth seed Elina Svitolina, with both women still able to reach the semi-finals heading into the final group matches.
Muguruza proved too strong in the Wimbledon final in July. but three months later it was Williams who prevailed in straight sets.
Both players struggled to hold serve in a see-saw opening set which was eventually won by Williams.
Leading 6-5, the American took advantage of a loose game from Muguruza in which the Spaniard drove a forehand long with the court wide open.
Before the second set, Muguruza's coach Sam Sumyk urged her to commit more to her shots, instructing the Wimbledon champion to "miss on your terms".
The words seemed to have an effect on Muguruza, who stepped inside the baseline to break in the opening game of the second set.
Muguruza could not sustain her advantage, however, and was pegged back instantly.
Williams took advantage of further errors from Muguruza to seal the match and reach the fifth WTA Finals semi-final of her career.
"I played an amazing opponent who has had an amazing year so I was so happy to win the last point," Williams said.
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.