LONDON (Reuters) - Roger Federer's hopes of winning a record-extending seventh ATP Finals title suffered a blow as Kei Nishikori stunned the Swiss great 7-6(4) 6-3 in their round-robin opener on Sunday.
The 37-year-old began calmly enough but his usually smooth game began to fray towards the end of the first set and fell apart in an error-strewn second.
Unsurprisingly the match ended with a Federer mishit, his 34th unforced error of a scrappy display.
Nishikori went 6-1 ahead in the first set tiebreak and although Federer clawed back to 6-4, a forehand into the net allowed the Japanese to take the opener.
Federer, who had qualified for the season-ender for the 16th time, cut a disgruntled figure at times and was even given a warning for swiping a ball high into the stands in the opener.
Nishikori, twice a semi-finalist in London, grew in confidence and broke in the sixth game of the second set for a 4-2 lead when Federer was left flat-footed.
Serving at 5-3 Nishikori might have expected a Federer fightback but it never materialised as he held serve easily to join Kevin Anderson at the top of the Lleyton Hewitt group.
(Reporting by Martyn Herman, editing by Pritha Sarkar)