LONDON (AFP) - Kei Nishikori gave his hopes of qualifying for the last four of the World Tour Finals a major boost with a 7-5, 3-6, 6-3 victory over Tomas Berdych on Tuesday.

The eighth-ranked Japanese wobbled in the second set but recovered to beat the Czech world number six thanks to what he admitted was a "lucky" shot at London's O2 Arena.

Both players lost their Finals group stage opener - Nishikori to Novak Djokovic and Berdych to Roger Federer.

A second defeat for Berdych left his semi-final hopes hanging by a thread.

Serbian world No. 1 Djokovic plays Swiss third seed Federer in Tuesday's other group match.

Victory for Djokovic would secure his place in the semis, while a straight sets win would guarantee Federer a last four spot.

Nishikori, the youngest player in the Finals at 25, made the semi-finals last year in his debut at the season-ending tournament, disputed between the top eight players on a slow-playing hard court.

Nishikori was receiving plenty of support from compatriots waving the Japanese flag and a clutch of well-refreshed British fans roaring him on.

The first set went with serve until Nishikori broke for 6-5 and then won the set on serve, having given Berdych plenty to think about with some clever drop shots and lobs.

Berdych was 2-0 down in the second set but turned it around, winning five games in a row.

In the third and deciding set, Nishikori saved a break point at 3-3, the key moment in the match.

"That was a really big point. He had three, four shots that he went really aggressive. I was defence all the time. Last shot, I went for it, closed my eyes. (It) was a little bit lucky I made that shot," he said.

Asia's top player then broke Berdych and closed out the two hours and 23 minutes-long match with a game to love.

The Czech also felt the missed break opportunity made all the difference.

"Everything was fine. I was playing very good in the second set. I was playing well in the third," the 30-year-old said.

"I was the one having a first chance having a break point in the third set. (We) had a good rally, (I) had a good position to finish the ball, but I just didn't make it. Then he took the chance. He broke me next game. That was it."

Nishikori said he was looking forward to Thursday's clash with Federer, the youngest player in the tournament facing the oldest.

"It's going to be a really big challenge for me to play Roger. It's always exciting," he said.

"It's going to be a really important match for both of us. So I'll try to recover well, and I will get good tactics ready and try to play another good match.

"He doesn't look really his age, 34. He's still improving himself, playing great tennis even though he's 34. He doesn't look old for me. It's a great honour to play Roger."

Even if Berdych beats Djokovic in his final group match on Thursday, his chances of making the last four would still be slim. Something that is not lost on him.

"Next match is against Novak. Another one to play with him, challenge him, just try to have a good one for the last match of the season," said Berdych.