Amelie Mauresmo was dealt a blow in her bid to finish the year atop the rankings when she lost to Nadia Petrova 6-2, 6-2 in opening round-robin play at the WTA Championships on Tuesday.
Meanwhile, Maria Sharapova and Justine Henin-Hardenne improved their chances of advancing out of the group stages and claiming the season-ending No. 1 ranking with wins.
To finish with the No. 1 ranking for the first time, Mauresmo needs to defend her title, hope No. 2 Sharapova does not make the final, and have No. 3 Henin-Hardenne finish last in her group.
"I speak better Spanish than I played today," Mauresmo said. "There's nothing to say about that match except it couldn't get worse in the next couple of matches. It's hard to say anything positive about a match like that."
It was Mauresmo's first match since withdrawing before the Zurich Open quarter-finals two weeks ago because of a right shoulder injury.
"There was no pain in my shoulder tonight -- I just feel a little behind the play and I made stupid mistakes," Mauresmo said. "It's the kind of match I would play after missing competitions."
The fifth-ranked Petrova broke the Frenchwoman five times on her way to an easy victory.
Henin-Hardenne will knock Mauresmo out of the race for No. 1 with another win after her 6-2, 6-7 (5), 6-1 victory over former champion Martina Hingis.
The French Open champion dominated Hingis with her backhand in taking the first set.
Serving at 5-2 in the second set, Henin-Hardenne allowed Hingis to rally by committing 25 unforced errors. Henin-Hardenne shook off the poor set and broke Hingis three times in the third set on her way to winning the match -- her first since tearing a muscle in her right knee in the doubles of the Fed Cup final against Italy in September.
Sharapova, who won the championships two years ago, broke Dementieva four times to take the first set in 27 minutes.
Neither player managed to hold convincingly in the second set, and Dementieva's serve -- which accounted for 15 double faults -- let her down again and Sharapova broke her for the seventh time to seal her ninth successive win and a 6-1, 6-4 win.
"I don't think I expected myself to play my best tennis tonight," Sharapova said. "I was rusty at times, but I hope I can learn a few things from today and move on and get better from here."
The 19-year-old Sharapova can finish the year as No. 1 should she win her sixth title of the year this week and Henin-Hardenne fails to reach the final.