Novak Djokovic sees off Nishikori to reach O2 final

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 U.S. Open runner-up Nishikori double-faulted to end the contest after one hour 27 minutes.

"Kei was playing great and this has been the best season of his life but he made some crucial double faults in the third set," seven-times grand slam champion Djokovic said.

"Of course I will watch the semi-final (between Roger Federer and Stan Wawrinka). I'll get some popcorn and enjoy it from my bed."

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.

When the pressure of the occasion appeared to get to Nishikori in a first set that last 23 minutes, the Djokovic juggernaut looked unstoppable.

Nishikori ballooned another forehand long to drop serve at the start of the second set and the crowd, who paid big money for their tickets, looked like being short-changed.

Things turned in the next game, however, when Nishikori suddenly had a break point chance and Djokovic double-faulted -- prompting huge cheers from the stands which clearly annoyed Djokovic who sarcastically applauded.

Suddenly world number five Nishikori began to dictate the rallies and drag Djokovic out of his comfort zone.

Leading 4-3 he set up a break point with a forehand winner and converted it after teasing Djokovic with a drop shot and then a topspin lob the Serb could only fend back into mid court where Nishikori ripped away another forehand winner.

Two aces helped Nishikori level the match and a grandstand decider looked in store after so many disappointing one-sided matches at the tournament.

However, you squander opportunities against Djokovic at your peril and when the Serb dug himself out of trouble to move 1-0 ahead in the third set, the momentum swung back his way.

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 claim 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.