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.''
German qualifier Mona Barthel and Petra Cetkovska of the Czech Republic also advanced to the second round. Barthel eased past Barbora Zahlavova Strycova of the Czech Republic 6-3, 6-2, and Cetkovska beat Italian lucky loser Alberta Brianti 6-3, 6-3.
