LONDON (REUTERS) - World No. 1 Novak Djokovic ended Kei Nishikori's impressive debut at the ATP World Tour Finals with a 6-1 3-6 6-0 semi-final victory at the O2 Arena on Saturday.

The Serb was rattled by a mid-match onslaught from the Japanese who came back from a poor start to dominate the second set, only for his challenge to fizzle out in the decider.

Things could have been different had Nishikori, the first Asian man to qualify for the year-ender, converted the two break points he had in the first game of the deciding set.

But Djokovic held firm and quickly regained control to march towards a 31st consecutive victory on indoor courts.

He set up a match point with a searing crosscourt forehand winner and US Open runner-up Nishikori then double-faulted to end the contest after one hour 27 minutes.

Djokovic had gone through the group stage with wins against Marin Cilic, Stanislas Wawrinka and Tomas Berdych for the loss of a mere nine games, a tournament record.

The 27-year-old, who has already clinched the year-end world number one ranking, can become the first man since Ivan Lendl (1985-87) to win three consecutive Tour Finals crowns if he wins on Sunday against either Roger Federer or Wawrinka, who face each other later in an all-Swiss semi-final.