Marin Cilic fights back to beat Kei Nishikori at the O2

The Japanese had already qualified as the runner-up in John McEnroe group following Andy Murray's earlier victory over Stan Wawrinka, and he will be thankful this loss wasn't decisive as he fell away 6-3 2-6 3-6 in the final round-robin match at the O2 Arena.

Nishikori was hoping the 200 points on offer for a victory would bolster his bid to finish the year as the world number three, but that now looks highly unlikely as he attempts to lift himself in time for his clash with Djokovic on Saturday evening.

Nishikori started brightly and made the early breakthrough in game three when he took advantage of a poor forehand error from Cilic to convert the break point with a low, angled crosscourt backhand.

The Japanese world number five looked confident as he dominated without overexerting, and he broke again on the second of three set points to seal it inside 38 minutes.

Nishikori would have wanted to finish the job quickly but his game fell apart in the second set. Cilic broke for 2-1 thanks to a pair of ill-timed malfunctions off his opponent's forehand, and the Croat snared the double break to love as Nishikori's first-serve percentage nose-dived.

Cilic had grown into the contest and his big serve was flowing, a fact exemplified by the pair of aces that saw him take his first set of the week from deuce.

Nishikori's unforced error count continued to rise and Cilic took advantage to earn a break point and convert it with a rasping backhand winner down the line in the fifth game of the decider.

The Croatian suddenly looked imperious and he broke again with the Japanese serving to stay in it, clinching his first win at this tournament at the sixth time of asking to move up to sixth in the world and ensure he leaves London for next week's Davis Cup final on a high.