Serena Williams was forced to retire from the Rogers Cup final in Toronto on Sunday after suffering from back spasms.

After beating Naomi Osaka and Marie Bouzkova in the quarter-finals and semi-finals respectively Williams started the final against Canadian Bianca Andreescu as the favourite to win the tournament.

However, Williams could only manage four games of the opening set before calling the doctor on-court and retiring in visible pain in her first final since her Wimbledon loss to Simona Halep. 

Her opponent, Andreescu, took the title in front of a home crowd and was the first to console Williams as she broke down in pain.

"I watched your whole career, you're a f---ing beast," she told Williams in a touching moment after the world number eight made the decision not to try and play through the pain.

Williams explained in her post-match press conference she had been struggling with back spasms towards the end of her semi-final match against the Czech, Bouzkova. The 23 time Grand Slam champion explained this was not the first time she had experienced a similar issue with her back.

“I don't want to get this far and not at least try. I think I would have really regretted not at least going out there and seeing maybe if a miracle happened.

"I've had this before and it's, like, 24, 36 hours where I'm just in crazy spasm and then it's, like, gone. And so that's a little frustrating for me because I know that I could play, I just can't play today," she said.

Williams' retirement in the first set opened the door for 19-year-old Andreescu's second title of the tour at her first tournament since bowing out of the Roland Garros French Open with injury in May.

The Canadian spoke of her admiration for Williams in her post-match speech to a home crowd in Toronto. 

"Serena, you made me cry. I know how it is to pull out of tournaments. It's not easy... this wasn't the way I expected to win. You are truly a champion. I've watched you win so many times. You are truly a champion, on and off the court. I'm speechless right now. This has been a dream come true."