TORONTO, Canada - 19-year-old Canadian Bianca Andreescu took down No.8 seed Serena Williams to lift the trophy at home at the Rogers Cup - but the highly-anticipated final unfolded in a manner that no one expected. 

Playing in her first WTA-level final since Rome 2016, a tearful Serena found herself unable to continue after just four games into the opening set, retiring from the match citing an upper back injury, trailing 3-1.

Read more: Hometown hero Andreescu claims Toronto title after Serena retires: 'You just have to listen to your body'

Afterwards, Serena told press that the troubles started the night before during her tough, three-set battle in the semifinals against qualifier Marie Bouzkova. During that match, she experienced a painful back spasm, but managed to pull through to win 1-6, 6-3, 6-3. 

But in the hours and the night that followed, Serena revealed, the injury progressively became worse and worse. 

“Just my whole back just completely spasmed, and to a point where I couldn't sleep and I couldn't really move,” Serena told journalists in her post-match press conference. 

“And so I was just trying to figure out, ‘How do you play a match where you have no rotation?’”

During those four tough games on Sunday, the three-time Rogers Cup champion Serena tried to answer that question and stayed toe-to-toe with Andreescu - hitting two aces and picking up a hold of serve along the way. But it was that same weapon that was most affected by the spasm: Serena faced break points in both of her two service games, made just 39% of first serves and was broken once. 

“It's difficult for me to serve, I mean, I could move, but I need my upper body,” Serena said. “My lower body was fine, but I just couldn't do anything with my upper body and, again, for this period of time. 

“I had so much treatment before, hours and hours of treatment. So I just knew. I knew I wasn't going to be able to continue.”

With a bit more time to recover, Serena mused, she probably would have been able to take the court feeling differently. But at a WTA tournament where matches are typically contested on consecutive days, there was just not enough time for the treatment to kick in. 

Serena Williams shares a laugh with Rogers Cup champion Bianca Andreescu. (Getty Images)

“That's the most frustrating part,” she said. “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 bit frustrating for me because I know that I could play. I just can't play today.

“I do different treatments. I take a day off. But obviously, I didn't have 24 hours or plus to take off.”

Read more: 'This past week hasn't been easy' - Bianca withdraws from Cincinnati after blasting to Rogers Cup trophy

Despite the pain - which Serena said makes it tough to even get out of bed, let alone play professional tennis - the 23-time Grand Slam champion remained determined to put on a good show in front of the Rogers Cup crowd.

“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.”

Ahead of the match, both Serena and Andreescu made no secret of how much they were looking forward to their first career match, a battle of age and experience versus youth and audacity.

Bianca Andreescu with Serena Williams after the Rogers Cup final. (Getty Images)

That mutual respect was reflected on the court when, after seeing a tearful Serena forced to retire midway through their opening set, Andreescu knelt down next to the 23-time Grand Slam champion to share some words of encouragement - a gesture that Serena definitely appreciated. 

“I just think I was really sad, and she made me feel a lot better, so that was really nice,” Serena said, calling Andreescu ‘an old soul’: “She's only 19. She definitely doesn't seem like a 19-year old in her words, on court and her game, her attitude, her actions… She’s just a fabulous personality.”