Tennis news - Roger Federer stunned by Kei Nishikori in straight sets

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 clinch the win.

Kevin Anderson beat Dominic Thiem in the earlier match in Group Lleyton Hewitt.