Nishikori came into his opening match of the round-robin stage having not beaten Federer in their six previous meetings, but the world number nine battled to a 7-6 (7-4) 6-3 win at the O2 Arena.

It adds to a remarkable second half of 2018 for Nishikori, who started the year playing ATP Challenger events as he came back from wrist surgery.

Inflicting a rare defeat on Federer is undoubtedly the highlight of the year for Japanese, the six-time winner of the end-of-season event well short of his usual standards as he began his bid for a 100th career crown.

Federer's troubles began in game four as a wayward forehand and another into the net allowed Nishikori to gain some momentum, although neither player was able to fashion a break point.

The serene backhand that has defined Federer's career looked to have kicked into gear by game seven, but putting Nishikori under pressure proved a task too far.

He did have a 0-30 advantage in the 12th but Nishikori battled back to force a tie-break, and mistakes from Federer handed his opponent the lead.

After no breaks of serve in the first set, Nishikori double-faulted in the opener of the second, but Federer gifted the advantage straight back as his out-of-character display continued.

The sixth game proved crucial as Nishikori converted a break chance to lead 4-2, his deep forehand proving too good for Federer's backhand.

Crucially for Nishikori he then served out two games with aplomb, not giving Federer a sniff of a chance as he blew Group Lleyton Hewitt wide open.