Andy Murray of Britain reacts during his tennis match against Roger Federer of Switzerland at the ATP World Tour finals at the O2 Arena in London November 13, 2014. [Photo/Agencies]

Nishikori, the U.S. Open runner-up, is one of the three debutants at the tournament. He was rapt to advance at Ferrer's expense.

"The final set was almost perfect," said Nishikori, who hit 41 winners and won 80 percent of his first-service points.

The Spaniard, who replaced the injured Milos Raonic at the last minute, took advantage of Nishikori's 18 unforced errors in the first set and made the decisive break in the 10th game when the Japanese player netted an easy smash.

But Nishikori recovered to break at the start of the second set and served out to even the set score, making it the first match at the tournament to go to three sets following eight one-sided encounters.

The fourth-seeded Nishikori won four consecutive games to start the third, and saved five break points in the sixth game before winning it with a drop shot.

Ferrer missed out on qualifying for the ATP Finals after his loss to Nishikori in the Paris Masters quarterfinals last month. He said stepping in for Raonic at the last minute did not pose a problem despite articular problems in his back.

"My condition was perfect today, Kei was just better," said Ferrer, the 2007 runner-up.

The seventh-seeded Raonic withdrew with a muscle injury after losing his two group matches in straight sets.