"I hurt my right calf muscle in the last game against Mona Barthel, so I don't know if you can get any more unlucky than that," Stosur said. "I think we only played another four points in that match. I felt something go in my calf and it was hurting a fair bit. Yesterday I took the day off practice and only did treatment, probably iced it about 10 times. This morning I thought I'd give it a shot, and tried to go out and warm up, and after about five minutes I knew there was no chance I could play, unfortunately.
"Today I'll talk to the physios and doctors and work out a good plan for how many days off I need and go from there. It's all a bit up in the air at the moment because I've never been through this type of injury, so it's a matter of wait and see, talking to the people who know, and reassessing each day."
Stosur was in her third WTA quarterfinal of the year and was looking to reach her first WTA semifinal of the year - and in addition to that, she had won both of her previous meetings against Kerber.
"It's really disappointing," Stosur explained. "I had a bit of a rough start to the year and feel like now my tennis has really picked up, and I've been playing quite well these last few days. I love the conditions here at the BNP Paribas Open and I love playing in front of this crowd, so to have to pull out without even being able to get on court is probably as bad as it gets. I think this is only the second time I've ever pulled out of a match in my career, and unfortunately there isn't anything I can do about it."
Stosur's memory remained very sharp despite the injury - it was indeed just the second time she has had to pull out of a WTA-level match, the only other time coming all the way back in 2007 at Memphis when she had to stop during the first set of her match with Ioana Raluca Olaru due to a virus. She does have one other retirement in her career, but that came at an ITF Circuit event way back in 2003.
The No.4-seeded Kerber's walkover over the No.7-seeded Stosur put her into her second semifinal of the year, having also gone that far at Sydney - she was stopped there by Dominika Cibulkova.
"For sure it's better when you play and win the match to go to the semis," Kerber commented. "I didn't know this until I was ready for the match. Sam just told me after I warmed up that she couldn't play. She was trying to warm up because she was coming after my practice. Of course it's bad luck."
While the situation gives Kerber an extra day of rest and recovery, in one way it could hurt her, in that she goes into the semifinals of one of the biggest tournaments of the year not having played anyone in the Top 30 - the highest-ranked opponent she has played is No.32-ranked Yanina Wickmayer.
"My rhythm now is a little bit different," Kerber said. "But I did my practice - I hit one hour after the situation came up - and now I'll have a day off to get ready for tomorrow. So it's okay for me.
"Tomorrow it starts again from zero. I will focus on my things and have fun and play my tennis."
This isn't Kerber's biggest WTA semifinal - among her previous 16 semifinals are two Grand Slams (2011 US Open and 2012 Wimbledon) and another Premier Mandatory (2012 Indian Wells). But she was stopped in all those matches, so if she were to make the final here, it would be her biggest final.