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London - Roger Federer made a strong start to his bid to cap his golden year with a seventh ATP Finals title as the Swiss star defeated Jack Sock 6-4, 7-6 (7/4) on Sunday.
In the other match in Federer's group, promising German youngster Alexander Zverev showed his potential with a 6-4, 3-6, 6-4 win against Croatia's Marin Cilic.
When Federer missed last year's Tour Finals through injury, it seemed possible he would never recapture his former glories.
He came into 2017 having missed half of last season due to fitness problems and was without a Grand Slam title since 2012.
But the 36-year-old has enjoyed a stunning renaissance, winning the Australian Open and Wimbledon among seven titles - more than other male player over the last 11 months.
Federer hasn't triumphed at the season-ending Tour Finals since 2011.
But after seeing off Sock in his group opener, the world number two has now won his last 11 matches.
That dominant streak has already earned him titles in Shanghai and Basel, and he hopes to maintain that hot streak all the way to the Tour Finals crown next weekend.
"It's wonderful to be back, especially after missing last year with injury," Federer said.
"I got off to a great start, but the second set was tight.
"The breaker could have gone either way and in the end he helped me with some mistakes. I'm really happy I got through somehow."
With defending champion Andy Murray, Novak Djokovic and Stan Wawrinka all absent from the Tour Finals due to injury, Federer and Rafael Nadal are the unquestioned headline acts.
Having won the French and US Opens this year, Nadal has matched Federer's unexpectedly dominant run as the old rivals hoovered up all four of this year's Grand Slam crowns.
Nadal is already guaranteed to finish as the year-end world number one for the first time since 2013, making the 31-year-old the oldest man to earn that honour.
But Nadal's hopes of finally winning the Tour Finals for the first time could be hampered by injury.
Nadal, twice a Tour Finals runner-up, is due to open his campaign against David Goffin on Monday, however the Spaniard admits he is still struggling with the knee problem that forced him to pull out of the recent Paris Masters.
Speaking after receiving his year-end number one trophy following the Federer match, Nadal said he is ready to give it a go against Goffin.
"It has been a fantastic season, a very emotional one after all of the things I have been going through in the last couple of years with injury. See you tomorrow on court," he said.
While Nadal frets over his fitness, Federer was back in the groove against Sock.
Even Sock, yet to make it past the last 16 at any Grand Slam, was caught off guard by his climb to a career-high ninth in the rankings.
He was forced to cancel a scheduled round of golf at Augusta as he instead prepared to face Federer.
A relaxing stroll around that famous course might have seem preferable to facing Federer, who maintained his record of never dropping a set to the American.
Fans hoping for some young blood to challenge Federer and Nadal got a glimpse of a possible future star as world number three Zverev made his tournament debut.
Zverev is the youngest Tour Finals qualifier since Juan Martin del Potro in 2008, the towering 6ft 6in 20-year-old having underlined his emergence with five titles this year.
He gave an impressive show against Cilic, coming from 3-1 down in the final set to beat the former US Open champion.
Collated results from the first day of the ATP Tour Finals at London's O2 Arena on Sunday (x denotes seeding):
Singles
Group Boris Becker
Roger Federer (SUI x2) bt Jack Sock (USA x8) 6-4, 7-6 (7/4)
Alexander Zverev (GER x3) bt Marin Cilic (CRO x5) 6-4, 3-6, 6-4
Doubles
Group Eltingh/Haarhuis
Ryan Harrison/Michael Venus (USA/NZL x8) bt Henri Kontinen/John Peers (FIN/AUS x2) 6-4, 7-6 (10/8)
Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x6) bt Jean-Julien Rojer/Horia Tecau (NED/ROM x3) 1-6, 7-6 (9/7), 10-8
