BEIJING, China - The last four quarterfinal spots were snapped up at the China Open on Thursday and one of the results was a major surprise, as Nadia Petrova upset Serena Williams in a marathon at night, 64 36 76(5).

Petrova, seeded No.13 at the $4.5-million tournament, split sets with the No.2-seeded Williams before the two put on a serving display in the third set, neither one losing a service game - and each only facing break points in one of their six service games - en route to the tie-break. It was the Russian who came out of the blocks strongest, racing out to a 5-0 lead, and she resisted the traditional Williams comeback and closed it out.  Shot of the Day: Nadia Petrova

"I had some opportunities I let slip through my fingers," Williams said after the match. "Had I capitalized on my opportunities, maybe I would've won. But now my goal is just to do well in Doha."

Williams needed to do better than Safina at this tournament to secure a return to the No.1 ranking next Monday and she did that by reaching the third round - Safina lost in the second round and will drop to No.2 when Monday comes.

Petrova had only won one of seven previous meetings with Williams but in her last two losses had pushed the American to three sets each time, both times losing 6-3 in the decider.  Interview: Nadia Petrova

In other matches, No.7 seed Vera Zvonareva gained revenge for a US Open round of 16 loss to No.10 seed Flavia Pennetta with a 63 57 75 win; No.12 seed Agnieszka Radwanska beat María José Martínez Sánchez, 46 64 62; and No.14 seed Marion Bartoli beat wildcard Zhang Shuai - conqueror of Safina in the second round - in straights, 61 64.  Interview: Marion Bartoli

Radwanska was down 64 40 against Martinez Sanchez but won 12 of the last 14 games to move through.  Interview: Agnieszka Radwanska

No.4 seed Elena Dementieva, No.6 seed Svetlana Kuznetsova, and unseeded players Anastasia Pavlyuchenkova and Peng Shuai won third round matches on Wednesday. Friday's quarterfinals pit Dementieva against Radwandksa, Kuznetsova against Pavlyuchenkova, Zvonareva against Bartoli and Petrova against Peng.  Interview: Vera Zvonareva