According to the Swiss media Stan strained the stomach muscle already on Sunday in the training and underwent an ultrasonic testing.
Stan said that the risk wasn't worth it to continue the match against Giquel. He would have felt the pain more after the second set and was handicaped during his serve.
He mentioned that he is really disappointed especially as he thought he had a good form. He will return to Switzerland for further medical check-ups.
Get well soon Stan