Ana Ivanovic was stunned 6-2, 1-6, 6-2 by 17-year-old Austrian Tamira Paszek in the third round of the Montreal Cup on Thursday, opening the door for fellow Serbian Jelena Jankovic to claim her number one world ranking.
Jankovic, reached the last eight with 6-3, 6-2 demolition of Canadian Stephanie Dubois, sits just 88 points behind Ivanovic in the WTA rankings and can nudge her out of top spot by making it to Sunday's final.
Playing her first event since a third-round exit at Wimbledon, Ivanovic displayed little of the form that had carried her to a French Open title in May and victory on the same Canadian hardcourt in 2006.
Even more shocking, the defeat came against an opponent who had won just three matches since the Australian Open and had gone five tournaments without a victory until a first-round win last week in Los Angeles. Paszek had never beaten a top-10 player.
Ivanovic said she had been affected by an inflamed right thumb that was painful enough for her to consider returning to Europe to have it examined by doctors instead of flying directly to the Beijing Olympics.
"I really hope not," Ivanovic told reporters when asked if her Olympic plans were in jeopardy. "I was in pain through whole match. I spoke to many doctors, and they all said it just takes time for inflammation to get out.
"Now obviously I'm a little bit scared because MRI shows nothing, but I still have pain. They say it's going go away, but it's there for already 10 days so I'm going to try and see the doctor now and try to get it out.
"I was planning to go straight to Beijing. But now I might go back to Europe to see the doctor."
If Ivanovic were to rule herself out of the Games, it would be another huge blow to the women's Olympic tournament following the withdrawal of world number three Maria Sharapova with a shoulder injury.
Meanwhile, world number four Svetlana Kuznetsova advanced to the quarter-finals of the Canadian event with a 7-5, 2-6, 6-4 win over 15-year-old Portuguese qualifier Michelle Larcher de Brito.
Seventh seed Dinara Safina, riding the momentum from her title win on Sunday in Los Angeles, raced into the last eight with a 6-2, 6-2 thrashing of Swiss ninth seed Patty Schnyder.
Safina will next face unseeded Slovakian Dominika Cibulkova, who beat 12th-seeded Russian Nadia Petrova 7-6, 6-2.
Tenth-seeded Frenchwoman Marion Bartoli also advanced after overcoming Russian sixth seed Anna Chakvetadze 4-6, 7-5, 7-6.
Japan's Ai Sugiyama was the first player through, advancing in a walkover when Sharapova pulled out of the tournament on Wednesday.