The world number one appeared to be suffering from a minor panic attack when she left the court for nine minutes after failing to convert five match points.
When asked whether her decision to call for the trainer was an act of gamesmanship, she replied: "No, it was necessary thing for me to do. I just regret that I didn't take it earlier.
"It got to the point that it was pretty much impossible for me to breathe and to play. It took a little bit longer. The timing, it was my bad. The game before that when I lost my service game, it kept getting worse.
"I thought I would have to play through it and keep calm. But it just got worse. I had to do it. I had to unlock my rib, which was causing my back problem. The trainer said we had to go off court to treat that. I just didn't really want to take off my dress on the court."