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.
More to follow.