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.
Andy Murray is targeting a New Year comeback in Australia but has stressed he will not rush a return before his body is ready.
The Scot has not played since struggling through Wimbledon with a hip injury but is hopeful he will be able to take part in the Brisbane International, which kicks off next season's action on December 31.
Murray is continuing his rehabilitation by limbering up for an exhibition match against Roger Federer at the Andy Murray Live in Glasgow on Tuesday evening and feels he is making good progress.
When asked about his chances of making Brisbane, he said: "I hope I'm there. Things have been going pretty well so far in the rehab, but you just never know. You take each week as it comes.
"I've had setbacks and things can go quite quickly as well. I've been training for a few weeks now. Some days I've felt great and some days not so good.
"But I will come back when I'm ready and when I'm 100 percent fit. I probably made a bit of a mistake trying to get ready for the US Open but it was the last major of the year and I wanted to give it a go.
"And now it's time to give my body the rest and recovery it needs. I'll come back when I'm ready."
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Novak Djokovic will make his return to tennis in Abu Dhabi at December's World Tennis Championship.
The Serbian has skipped the back end of the current season because of an elbow injury, playing no part since abandoning his Wimbledon quarter-final against Tomas Berdych in July.
But 30-year-old Djokovic will return for the 2018 campaign, and the 12-time grand slam winner is planning an early start with an appearance at the December 28-30 exhibition tournament in the United Arab Emirates.