Samantha Stosur has scored a rare win over Maria Sharapova, a spot in the Pan Pacific semi-finals and improved her chances of featuring in the WTA's end of season event after a big afternoon in Tokyo on Thursday.

Holding a 10-1 career record against the Australian, Sharapova was a heavy favourite in their quarter-final but Stosur emerged a 6-4 7-6 (12-10) winner over the two-time champion.

Stosur broke Sharapova to go up 5-3 in the second set but then lost nine straight points as Sharapova sent the match to a tiebreaker, which Stosur won when Sharapova's return was long.

The win boosts the world No.9's chances of an appearance at the WTA Championships in Turkey which is limited to the top eight ranked players in the world.

With current world No.8 Li Na losing to Caroline Wozniacki in a third round match and Stosur only having to defend points from making the last 32 of the tournament last year, the 2011 US Open champion is still well in the reckoning for the prestigious event.

"Obviously she was up 5-3 and had a chance to win the match comfortably. I had chances in the tie-break (to win the set). Just a one point here, one point there changed a little bit, but it didn't go my way," said Sharapova.

"I could have returned better, but she played a solid match today. She was much more consistent. It's disappointing to lose."

Meanwhile, Stosur said: "It was tough. I knew I lost nine points in a row (at 5-3 in the second set). I just tried to start well in the tie-breaker and I did get off to a good start. I just tried to keep going forward."

"In the tie-breaker, I had match points and she had set points. You don't get a few in a row and you don't know what happens. I just tried to keep going forward."

Stosur will face Nadia Petrova of Russia on Friday for a spot in the final.

Petrova beat sixth-seeded Sara Errani of Italy 3-6 7-5 6-3.

Sharapova wasn't the only top player to bow out.

Top-ranked Victoria Azarenka withdrew after a bout of dizziness in her third-round match on Wednesday, advancing Angelique Kerber of Germany to the semis.