Blake, the bottom seed and last player to qualify, needed seven attempts to hold serve before he found his rhythm in the second set and broke late for the third.

It was Blake's second win over a high-ranking opponent here after he shocked number two Rafael Nadal in straight sets in his opening round-robin match.

Blake, who stretched his record against Davydenko to 5-0, will qualify for the last four if Nadal beats compatriot Tommy Robredo in Wednesday's later Gold Group match.

Davydenko said he was ill-prepared for the match after coming down with a virus.

"I wasn't ready. My tennis is good but physically I wasn't ready," he said.

"It's not because I never beat Blake. I felt like I should have beaten him today because I was playing well.

"I think if I played faster I would have had a good chance."

Davydenko was the first to find form in an error-strewn opening set when neither player was able to hold serve for six games.

The Russian finally held for 4-2, whipping a forehand past Blake as he came to the net, and broke again before serving out.

Blake went down another break early in the second before eventually holding, sparking the revival as he broke for the first time for 4-4 and then again to tie the match at a set each.

The players exchanged breaks in the third set before Blake converted his third match point, breaking for 7-5 as Davydenko misfired with a forehand.

Blake had won all four previous matches with Davydenko.

He qualified for the tournament on the back of a breakthrough season in which he snared five titles and entered the top 10 for the first time.

Davydenko is also at a career-high ranking after a globe-trotting year in which he played 31 tournaments before the Masters Cup.

He rose to number three after winning last week's Paris Masters and will face Nadal in his final group match.

The world's top eight players are competing in the 4.45-million-dollar, season-ending tournament, which is in a round-robin format ahead of Saturday's semifinals.