Serena Williams was weeping from the sidelines after having to back out of a big match Sunday -- but her opponent proved to a good sport ... and her biggest fan.
The tennis icon was facing 19-year-old Canadian phenom Bianca Andreescu in Toronto for the 2019 Rogers Cup Final -- and after playing four sets ... Serena was forced to pull the plug and remove herself from contention due to back spasms she was experiencing.
According to reports, she had started to feel the spasms as late as Saturday night during her semifinal victory over Marie Bouskova.