Roger Federer's hopes of winning a record-extending seventh ATP Finals title have suffered a blow as Kei Nishikori stunned the Swiss great in their round- robin opener in London.
Federer 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 to go down 7-6 (7-4) 6-3.
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 tiebreaker 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.
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.
Earlier Anderson made a winning debut at the ATP Finals with a 6-3 7-6 (12-10) victory over Austrian Dominic Thiem.
Wimbledon runner-up Anderson, the first South African to qualify for the season-ender since Wayne Ferreira in 1995, was comfortably the better player in a one- sided first set.
The second set was a much tighter affair with serve dominating but Anderson won a gripping tiebreaker 12-10 on his fourth match point, banging down his 13th ace.
Anderson, 32, celebrated by leading the packed crowd in singing 'happy birthday' to his wife Kelsey.
The Gustavo Kuerten Group starts tonight with world number one and tournament favourite Novak Djokovic playing American john Isner and Marin Cilic playing Alexander Zverev.
New Zealander Michael venus and his South African partner Raven Klaasen were beaten in their opening doubles match by Jamie Murray and Britain and Brazlian Bruno Soares.