Jelena Jankovic was upset 7-5, 6-2 by Slovakia's Dominika Cibulkova in the quarter-finals of the Montreal Cup on Friday, ending her chance of replacing fellow Serb Ana Ivanovic as world number one this week.
Following a shock third-round exit on Thursday, Ivanovic had left the door open for world number two Jankovic to nudge her out of top spot by reaching Sunday's final.
But for the second consecutive week, Jankovic failed to seize her opportunity, suffering a stunning centre court collapse at the hands of the 19-year-old Slovak.
"It doesn't matter, the first ranking. I want to be healthy," Jankovic told reporters. "I don't really think about number one.
"If it's going to happen, it will happen but at the moment I don't deserve that spot. I am not in the best shape."
Jankovic, whose bid for the number one spot last week at the Los Angeles Classic was ended with her loss to Dinara Safina in the semi-finals, had looked poised to race in to the last four after taking a 5-1 lead in the opening set.
But she suddenly lost her way, Cibulkova sweeping the next eight games to take the opening set and a 2-0 lead in the second.
Growing more frustrated with each missed shot, when Jankovic did find the mark the ball was returned with venom by the hard-hitting Cibulkova, who ended a miserable afternoon for the Serb by closing out the match with another break.
"Until 5-1 I was doing quite well," said Jankovic. "I was feeling good and I was hitting the ball well. But then all of a sudden I just got exhausted, completely lost my concentration.
"I'm still not in the best physical shape...I felt dizzy on the court. I needed a little bit of time after the match to come to life again. I'm still not feeling that strong."
The 31-ranked Cibulkova said: "In the first games I was nervous and made a lot of mistakes. Then I found my rhythm and started believing in myself."
Cibulkova faces Marion Bartoli in Saturday's semi-finals after the 10th-seeded Frenchwoman advanced with a 6-2 6-3 win over Japan's Ai Sugiyama.
In the other quarter-finals, world number four Svetlana Kuznetsova faced seventh seed Safina in an all-Russian showdown while 17-year-old Austrian Tamira Paszek, who upset Ivanovic, was due to play 11th seed Victoria Azarenka of Belarus.