|Venue: O2 Arena, London Dates: 11-18 November|
|Coverage: Follow live coverage across BBC TV, radio, the BBC Sport website & mobile app. Live text commentary available on selected matches.|
Roger Federer paid for his errors as he lost to Japan's Kei Nishikori in his opening group match at the ATP Finals.
The Swiss, chasing a 100th career title at the season-ending event, lost 7-6 (7-4) 6-3 to the world number nine.
The normally unflappable Federer showed uncharacteristic frustration, while a calm Nishikori sealed victory when the Swiss hit a wild forehand.
World number one Novak Djokovic opens his campaign against John Isner at the O2 Arena in London on Monday.
Federer hit 34 unforced errors - and was given a warning by the umpire for swiping a ball into the stands in the first set - in a performance that he will be keen to forget.
The 37-year-old will be back in action on Tuesday when he faces Dominic Thiem, who lost to Kevin Anderson earlier on Sunday.
It is the fourth time Federer has lost his opening match at the event, but he can take some comfort from the fact that in 2007 he still went on to win the title.
Federer began the match calmly and he put pressure on the Nishikori serve in the fourth game, taking him to deuce. But as his errors began to mount, his mood visibly soured.
Nishikori looked like he would run away with the tie-break, forging a 6-1 lead before Federer clawed it back to 6-4, but the Swiss then sent a forehand into the net to hand the set to the Japanese player.
The 20-time Grand Slam champion grew increasingly frustrated with himself, while Nishikori simply grew in confidence.
Having traded early breaks in the second, the Japanese player broke in the sixth game to go 4-2 up and then served to love in the following game.
More Federer errors gave Nishikori three match points and one final mis-hit forehand way beyond the baseline handed him victory.
"I'm glad to win, it is never easy to play with my idol, it is always a big challenge against him, so it was great to win," said Nishikori, who has reached the semi-finals here twice.
|Group Lleyton Hewitt|
What went wrong for Federer?
Federer pointed to a combination of nerves and the surface for his below-par showing, which followed a good run of results where he had won the title in Basel and reached the semi-finals in Shanghai and Paris.
"The court here plays differently, definitely slower than the last few tournaments I've played. So everyone is making minor adjustments, including me," he said.
"I've been feeling fine, just practice was a bit all over the place, practising at Queen's, practising on outside courts here and at Centre as well, so it was not always the same conditions.
"Overall I'm hitting the ball alright, the warm-up was fine.
"Maybe both of us had a bit of nerves too, not knowing how to attack second serves, trying to get the right feel for it. We both struggled to get that early."
Federer is chasing a record-extending seventh title at the tournament, having last won the title in 2011.
His chances had been improved by the withdrawal through injury of world number two Rafael Nadal and number four Juan Martin del Potro, but the favourite for the title before a ball had even been hit was Djokovic.
And as if to hammer the point home, the Serb appeared at the O2 Arena on Sunday afternoon to collect his trophy for sealing the year-end world number one ranking sporting a golden number one on his white top.
Former British number one Tim Henman on BBC TV
The way Federer has played throughout his whole career, he has made it look ridiculously easy. This was not easy and it's a shock for people to see.
I said before the match, for Nishikori to win, Federer had to play way below par and that's what happened.
Federer never found his rhythm from the back of the court. Nishikori is not a great server, but Federer didn't make a break point in the first set.
Time and time again we saw errors from the back of the court and not getting his feet into the right areas.
BBC tennis correspondent Russell Fuller
Federer had won his last 11 round-robin matches at the ATP Finals, but produced an erratic performance which hugely complicates his passage to the semi-finals. After Nishikori recovered from 5-6 0-30 down with the help of an extraordinarily athletic backhand half volley to win the first set on a tie-break 7-4, he was the stronger and more confident player.
Serving and volleying at opportune moments, Nishikori ended a run of six consecutive defeats against Federer.