Third-seeded Li took a medical timeout while trailing 7-6 (5), 3-2 to have her lower back massaged, and she 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 suffered last week in Serbia's 3-2 win over Belgium in the Fed Cup.
"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, Petra Martic of Croatia and Chanelle Scheepers of South Africa also advanced to the second round along with three German players -- Julia Goerges, Angelique Kerber and Mona Barthel.
Kerber, 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.