(CNN) -- Ana Ivanovic continued her recent dismal form by making a second round exit to Russia's Nadia Petrova at the Pan Pacific Open in Tokyo on Thursday.
A determined Petrova proved too strong for Ivanovic in the deciding set in Tokyo.
Petrova, currently 20th in the rankings, defeated the Serbian 6-11-6 6-2 in the tournament's first major upset.
Ivanovic has suffered a string of defeats since reaching world number one after winning the French Open and this was her latest setback.
"I tried to dominate but I was making too many mistakes," Ivanovic told the Associated Press. "I played better in the second set but she served well in the third and was able to play at her pace."
Ivanovic's Serbian counterpart Jelena Jankovic earlier coasted to a 6-2 6-1 win over Italy's Flavia Pennetta. If she wins this tournament, Jankovic would move back to top spot in the WTA rankings, replacing U.S. Open champion Serena Williams who is not playing in Tokyo.
"It's a goal of mine to return to the top spot," said Jankovic. "You can go back and forth between the top three spots, but it's always nice to finish at No. 1."
Jankovic will face Svetlana Kuznetsova in Friday's quarterfinal. The fifth-seeded Russian defeated Japan's Ayumi Morita 6-1 6-1.
In other action, Olympic gold medalist Elena Dementieva of Russia swept aside France's Alize Cornet 6-0 6-3 to reach the quarterfinals. Victory for the third-seeded Dementieva earned her a place in the WTA's season-ending tournament.
She joined Jankovic, Serena Williams, Dinara Safina and Ivanovic in qualifying for the November 4-9 championships in Doha, Qatar.
Dementieva, the winner of the Pan-Pacific tournament in 2006, had the first 10 games on the board before Cornet salvaged a measure of respectability.
Promising Polish teenager Agnieszka Radwanska is also through after seeing off Marion Bartoli of France 6-2 6-3.
In other WTA action on Thursday, top seed Vera Zvonareva of Russia beat China's Xu Yifan 6-1 7-6 to move into the quarterfinals of the Guangzhou International in southern China.
Zvonareva, a bronze medalist at last month's Olympics, will next face Italy's Karin Knapp, who beat eighth-seeded Mariya Koryttseva of Ukraine on Wednesday.
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