CARLSBAD, Calif. (AP) -- Jelena Jankovic had hoped to use the hardcourt season to prepare herself for the US Open.
Instead, the world's No. 2-ranked player was left with doubt after a 7-5, 6-2 loss to Alisa Kleybanova on Wednesday night in the second round of the Mercury Insurance Open.
Jankovic, who has not played since suffering an ankle injury two weeks ago, looked like she would have little trouble with Kleybanova when she broke the Russian's serve in the first and third games of the first set, and held her own serve for a 4-0 lead.
But Jankovic unraveled at that point as the 28th-ranked Kleybanova picked up her intensity and won 11 of 12 games to capture the first set and jump out to a 4-0 lead in the second.
"I wasn't really worried about it,'' Kleybanova said. "You don't have to get panicked. I said to myself to keep trying to find the key and pick up the pace.''
The defeat left Jankovic blaming her sprained left ankle for getting bounced out in her first match.
"I had some fear moving, at times in doubt because I'm afraid to twist it again,'' she said. "I'm still not confident 100 percent in the matches when I have to move side to side.''
Jankovic was playing in her first match since she was forced to retire during the Slovenia Open with a left ankle injury two weeks ago.
But she insisted despite not a lack of practice time and uncertainty about her ankle, it was important for her to return to the court to prepare for the US Open.
"I need to train and I need to get back in shape,'' Jankovic said. "It's tough to play when you're not in shape. I was wishing to play as many matches as possible and get match tough and be ready for the Open.''
Kleybanova broke Jankovic to tie the first set at 5-all before holding her own serve and winning the set when Jankovic hit a backhand into the net.
"I had to get used to her game and also to settle down my game, too,'' Kleybanova said.
Kleybanova has four career wins against Top 5 players but never any ranked as high as Jankovic.
Kleybanova broke Jankovic's serve in consecutive games to start the second set as she went in front 4-0. Jankovic, the top seed who received a first-round bye, suffered her first opening-match loss since January when she dropped a first-round match to Agnes Szavay at the Medibank International.
French Open finalist Samantha Stosur advanced to the quarterfinals with a 6-4, 6-4 win over American teenager Melanie Oudin.
Stosur, the No. 2 seed, used her powerful serve to control Oudin. The Australian also broke Oudin's serve late in both sets en route to her second-round win.
"I felt like at the start I wasn't timing the ball as well as I wanted to,'' Stosur said. "But as the match went on, I felt like I played better and better. I dictated and thought I served well. Overall, I was very happy.''
Stosur, who had a first-round bye, is coming off a semifinal showing last week at the Best of the West Classic, where she lost to eventual champion Victoria Azarenka. Oudin lost to Azarenka last week in a second-round match.
"I'm feeling confident,'' Stosur said. "I think I'm going along the right path to, hopefully, having a good US Open.''
Oudin, who made a run to the US Open quarterfinals last year, never really got into the match against Stosur, the world's No. 5 player.
"I thought I played pretty well,'' said Oudin, ranked No. 45. "She's 5 in the world and I was right in there with her the whole time. I'm not going to get down on myself.''
Oudin broke in the fourth game of the first set for a 3-1 lead. But Stosur took control with consecutive breaks against Oudin's serve plus holding her own serve twice to go ahead 5-3. In the sixth game, Stosur served consecutive aces, including one at 113 mph.
"Sam is pretty much an extremely tough opponent for me, especially because of her kick serve and her serve,'' Oudin said. "It's really, really tough to break her. It puts a lot of pressure on my serve because I know that I have to hold.''
Stosur, who lost to Francesca Schiavone in this year's French Open finals, used the only break of the second set to take a 4-3 lead.
In other second-round matches, fourth-seeded Agnieszka Radwanska of Poland defeated Russia's Dinara Safina 6-1, 6-3, and No. 7 seed Shahar Peer of Israel advanced with a 7-5, 6-4 win over Kazakhstan's Yaroslava Shvedova.