Victoria Azarenka began 2013 by making the tennis world wonder why she was walking off court in her semifinal against Sloane Stephens in Australia. She ended the year making us wonder why she was staying on it in Istanbul.

Serving at 2-3, 0-40 in the first set against Li Na on Friday, Azarenka stiffened up with what appeared to be an injury to her lower back. After winning the next point, she walked, gingerly, to the sideline and eventually off court with the trainer. When she came back, though, Vika could hardly move any better. She spent the next set and a half essentially waving at balls that Li hit past her. Azarenka was in obvious pain and had virtually no chance of qualifying for this weekend's semifinals, which only made her stubborn refusal to quit more puzzling.

As TENNIS.com’s Matt Cronin pointed out during the match, Vika has withdrawn or retired from 27 tournaments in her career, and this year alone she’s had injuries to her toe, foot, ankle, knee, lower back, and hip. Perhaps today she wanted to defy her reputation for fragility. If so, Azarenka can’t win either way: This was one situation when everyone watching would have understood why she had retired.

As for the pre-injury past of the match, Li started well, cracking six backhand winners in the first five games and going up 3-0. But Azarenka had shown some signs of competitive life; she broke back to make it 2-3 before pulling up lame.

Li said afterward that she didn’t like winning this way, but it was a day of triumph for her. She was hitting the ball well against the uninjured Azarenka. She kept her cool while facing a wounded opponent, which can often be a tricky psychological proposition. And, at age 31, she finished the steadiest year of her career by advancing to the semifinals of the WTA Championships for the first time. There she’ll play the winner of today’s match between Angelique Kerber and Petra Kvitova.