Novak Djokovic's Australian Open participation is in doubt after he was forced to withdraw from an exhibition event in Abu Dhabi with an elbow injury.
The Serbian has been out for almost six months and had been due to make his return to action at the World Tennis Championship.
However, he has now pulled out of a scheduled match against Roberto Bautista Agut and admits the injury "might affect the start of the season", with the Australian Open starting on January 15.
Andy Murray, who had been at the event just with the intention of training as he recovers from his own injury problems, will now step in to face Bautista Agut, according to a post on the tournament's Facebook page.
World number one Rafael Nadal's ongoing knee injury has seen him pull out of the season-opening Brisbane International.
Nadal was forced to withdraw from the ATP World Tour Finals in November with the issue and he has been unable to sufficiently recover in the off-season.
The Spaniard was due to play in the Mubadala World Tennis Championship in Abu Dhabi this weekend but pulled out earlier this week and he has now cancelled his plans to play in Brisbane.
However, he hopes to be fit for the Australian Open in Melbourne.
He said on Twitter: "I am sorry to announce I won't be coming to Brisbane this year. My intention was to play but I am still not ready after last year's long season and the late start of my preparation.
"I had a great time there and it was a great start to the month I spent in Australia.
"I will be seeing my Aussie fans when I land on the 4th in Melbourne and start there my preparation for the Australian Open."
British number one Andy Murray is due to play in the tournament as he embarks on his comeback from a hip injury.
Serena Williams will make her playing comeback following the birth of her daughter at the Mubadala World Tennis Championship in Abu Dhabi.
Williams has not played since winning the Australian Open in January, while she was six weeks pregnant, but will return to court to play French Open champion Jelena Ostapenko next Saturday.
The 36-year-old, who gave birth to Alexis Jr in September, is planning to make a competitive return at the Australian Open next month.
"I am delighted to be returning to the court in Abu Dhabi for the first time since the birth of my daughter in September," Williams told the tournament's official website.
"The Mubadala World Tennis Championship has long marked the beginning of the men's global tennis season and I am excited and honoured to be making my comeback as part of the first women to participate in the event.
"I look forward to seeing the fans in Abu Dhabi at the 10th Edition of the Championship very soon."
Men's world number one Rafael Nadal was also supposed to play in the event but withdrew due to ongoing troubles with his knee.
Former Wimbledon champion Marion Bartoli has announced her plans to make a shock comeback in 2018.
The 33-year-old Frenchwoman called time on her career in 2013, little over a month after lifting her only grand slam title at SW19, but intends to return to the WTA Tour next year.
Bartoli, who has been working as a commentator for Eurosport and ITV since her retirement, released the news on Eurosport's Twitter feed and hopes to be ready for the Miami Open in March.
She said in a video: "I am coming back this year on the professional tour. I still have a lot of practice ahead of me but I am hoping to be ready for March and the Miami Open.
"I am really looking forward to being on the court again and feel your support, especially at Roland Garros at the French Open and also Wimbledon, I am so looking forward to it."
She added on her Instagram page: "I am very happy to announce my return to the professional circuit of the @wta I can not wait to meet you and share great emotions with you again."
Bartoli suffered with health issues during her retirement and said in 2016 that she "feared for her life" after an unknown virus before eventually making a full recovery.
She enjoyed a career high ranking of seventh in 2012 and has lifted eight WTA singles titles. But her Wimbledon triumph - she beat German Sabine Lisicki in the final - is her only grand slam success.
British number two Aljaz Bedene will return to representing Slovenia from January 1, he announced on Facebook.
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The 28-year-old, who lives in London, became a British citizen in March 2015 and among home players the world number 49 is only behind Andy Murray in the men's professional rankings.
He has fought an unsuccessful battle to be eligible for Britain in the Davis Cup, and he admits that a desire to compete in that event, plus the Tokyo Olympic Games in 2020, played a big part in this latest move.
Lleyton Hewitt is coming out of retirement to play doubles at next month's Australian Open.
The former world number one officially called it quits after his home grand slam in 2016 but returned to the court twice more last year in Davis Cup and at Wimbledon.
Now he is making another appearance, this time alongside fellow Australian Sam Groth, for whom this will be the last tournament of his career.
Hewitt said in the Herald Sun: "It's going to be a bit of fun - that's what the Australian Open is about.
"I'm going to really enjoy it. We've been hitting a lot of balls, hitting every day, and we're not just going out there making up the numbers. We want to give it a fair crack."
Hewitt won grand slam singles titles at the US Open in 2001 and Wimbledon the following year, while he was also victorious in doubles at Flushing Meadows in 2000 partnering Max Mirnyi.
The 36-year-old is currently Australia's Davis Cup captain.
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Roger Federer's comeback year looks set for a final flourish as he began his bid for a record-stretching seventh ATP Finals crown with a comfortable win over Jack Sock. The 36-year-old beat American Sock 6-4 7-6 (7/4) in an hour and a half.
He hit 28 winners without offering Sock a single break point and while the 19-time major champion was not as clinical in the second set as he might have been, victory was never really in doubt.
Sock, making his first appearance here and yet to go further than the fourth round at a grand slam, took a while to find his rhythm and Federer pounced with a break in the very first game.
A punching backhand winner was followed by a dipping forehand to Sock's feet before another singing backhand gave Federer the early initiative.
Sock gradually settled his nerves as the set wore on and he had a sniff of a break back at 3-2 but a Federer ace and then a brilliantly picked up half-volley averted the danger.
Sock held serve for 5-3 in the strangest of fashions after he turned away and ducked his head to allow his opponent the entire court to finish off a short forehand. Instead, Federer lashed it into the net before both players shared a sheepish smile.
Federer was less generous moments later as an ace secured the set but Sock maintained momentum in the second frame, which was tighter than the first.
Sock had to fight hard to keep it close, defending five Federer break points, including two at 4-4, to force a tie-break. A disgruntled Federer pulled 4-2 clear but again Sock came back, with a successful challenge to make it 4-4.
Sock blinked at the crucial moment, however, as a double fault put Federer back in charge and he converted his first match point when a Sock forehand dropped long.