London - 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 suffered a wobble in the second set but recovered to beat the Czech world number six in two hours and 23 minutes on the hard court surface at London's O2 Arena.
Nishikori went into the match with a 3-1 career lead over Berdych, though the pair had not played since 2012.
Both players lost their Finals group stage opener -- Nishikori to Novak Djokovic and Berdych to Roger Federer, meaning a second defeat for either player would leave their semi-finals hopes hanging by a thread.
Serbian world number one Djokovic plays Swiss third seed Federer in Tuesday's other match in the group. Victory for Djokovic would secure his place in the last four, while a straight sets win would guarantee Federer a semi-finals spot.
Nishikori, the youngest player at the Finals at the age of 25, is making his second straight appearance at the season-ending tournament, disputed between the top eight players of the year, after reaching the semis last time out at the O2 Arena.
Nishikori was receiving plenty of support from compatriots waving the Japanese flag and a clutch of well-refreshed British fans roaring him on.
He gave Berdych plenty to think about with some drop shots and lobs but the Czech was proving a gritty opponent able to hold his service games under pressure.
With the first set going with serve, Berdych pushed Nishikori to deuce when the Japanese was serving to stay in the set but Asia's top player clung on to level at 5-5.
When Berdych double-faulted in the next game, Nishikori took advantage and went on to break, then won the set on serve with a forehand down the line.
Berdych was 2-0 down in the second set but managed to turn it around, converting both his break point chances.
In the third and deciding set, Nishikori fashioned two break points in the eighth game and clinched it when Berdych hit a return into the net, leaving the Japanese serving for the match at 5-3 up.
Nishikori took the next game to love, winning the match with a passing shot down the line.
Throughout the match, Nishikori's serve proved the more reliable and he made fewer unforced errors.
Berdych's hopes of reaching the last four are now hanging by a thread. He must beat Djokovic in his final group match on Thursday to have any chance of making the last four.
In the doubles competition, played before the singles match in each session, India's Rohan Bopanna and his Romanian partner Florin Mergea booked their place in the last four.
The eighth-ranked side beat home favourite Jamie Murray of Great Britain and Australia's John Peers 6-3, 7-6 (7/5), the fourth-ranked pairing.