STUTTGART, Germany — Serena Williams' game collapsed after she swept the first set today, leading to a 0-6, 6-1, 6-4 loss to China's Li Na that will cost her the No. 1 ranking.

By dropping the second-round match at the Porsche Grand Prix, Williams will be overtaken by Jelena Jankovic in Monday's new rankings, the WTA said. Williams gained the top spot when she beat Jankovic in the U.S. Open final three weeks ago for her ninth Grand Slam title.

Jankovic plays her first match in the tournament Thursday against Alona Bondarenko of Ukraine.

"Obviously, I want to be No. 1 but I am not focusing on that. I am focusing on winning titles and that goes hand in hand (with getting No. 1)," Williams said.

"I am not worried. It will come again if I win tournaments. I play to win titles and Grand Slams, although obviously the No. 1 is also on my mind."

Williams was playing her first match since the U.S. Open, getting a first-round bye in Stuttgart. She rolled through the first set and appeared headed for a resounding victory.

"I started making a lot of errors and I just gave it to her," she said. "Maybe I started doubting myself."

Li, ranked No. 30, said she was so nervous at the start of the match her hands were cold.

"I didn't have a chance in the first set," Li said. "But I looked at the clock and I saw it was only 20 minutes (into the match) so I told myself I could be winning in an hour. I knew I just had to hang in there."

Li also beat sister Venus Williams to reach the semifinals at the Beijing Olympics. Venus Williams is still in the Stuttgart tournament and watched Serena's defeat.

Li wasted three match points at 5-3, then fell behind 0-40 in her next service game after Williams had held for 5-4. She then put in a service winner and followed with an ace. After successfully challenging a call that her serve was out to get it to deuce, she clinched the match with her third ace of the game.

Li missed 11 months with a rib fracture last year and right-knee surgery in March.

"I believe I can go into the top 10 if I stay healthy," she said.

Fourth-seeded Elena Dementieva, the Olympic gold medalist, advanced to the third round by beating Sybille Bammer 6-1, 6-4.

Victoria Azarenka downed No. 8 Agnieszka Radwanska 6-1, 7-5 to reach the third round. Patty Schnyder beat Svetlana Kuznetsova for the first time in more than three years, gaining the second round with a 6-4, 4-6, 7-5 victory over the fifth-seeded Russian.

Schnyder is seeking her second title of the year after winning in Bali. Kuznetsova was coming off a runner-up finish at the China Open. It was her fifth final defeat of the year, with no titles. She was ranked No. 2 a year ago.

Seventh-seeded Vera Zvonareva made it to the second round by beating Daniela Hantuchova 6-3, 6-2.