PARIS (AP)—French Open champion Li Na retired with a sore back against Tsvetana Pironkova in the second set of her opening match Tuesday at the Open GDF Suez.

Third-seeded Li took a medical timeout while trailing 7-6 (5), 3-2 to have her lower back massaged and played a few more points before retiring.

“It was so painful,” Li said. “That’s why I called the physio to come to the court. I tried to tape and then tried to play the point, but it didn’t work. I’ll go to the hospital to take an MRI to see what happened.”

Fourth-seeded Jelena Jankovic withdrew Tuesday because of a left thigh strain she picked up in Serbia’s 3-2 win over Belgium in the Fed Cup last week.

“I was playing against (Kirsten) Flipkens and during one point in the second set, I felt a really sharp pain, but I managed to finish the match,” Jankovic said.

Li rallied from a 5-0 deficit to force a tiebreaker but she double-faulted at 3-2 to give Pironkova a mini-break. The unseeded Bulgarian won the first set on a forehand error from Li before taking a 3-1 lead in the second.

“I played last week in the Fed Cup in Israel, I played some good matches which helped for my form and for my confidence,” Pironkova said. “So I felt pretty good coming here, and I’m glad I could show it on the court today against such a world-class player.”

Petra Cetkovska of the Czech Republic and Chanelle Scheepers of South Africa also advanced to the second round along with three German players—Julia Goerges, Angelique Kerber and Mona Barthel.

Goerges cruised past Shahar Peer of Israel 6-1, 6-3. Kerber beat Lucie Safarova of the Czech Republic 6-2, 7-6 (3).

The U.S. Open semifinalist won five straight points to lead 5-1 in the tiebreaker, clinching victory when Safarova’s backhand return sailed long. Barthel eased past Barbora Zahlavova Strycova of the Czech Republic 6-3, 6-2.

Cetkovska defeated Italian lucky loser Alberta Brianti 6-3, 6-3, and Scheepers ousted Polona Hercog of Slovenia 6-2, 6-2.