Serena Williams had to retire from a tournament due to a back injury, but she received comfort from an unlikely source.

The tennis GOAT left the Rogers Cup women’s singles on Sunday due to upper back spasms, according to CNN. Williams played through the pain on Saturday, but the spasms worsened overnight. She eventually called a medical timeout and decided to retire.

"My whole back just completely spasmed, and to a point where I couldn't sleep and I couldn't really move," Williams explained.

The spasms are an ongoing issue and usually go away within a couple of days however, the tournament’s schedule prevented Williams from taking a little time off to recuperate.

"That's the most frustrating part,” she continued. “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."

Her forfeiture meant her opponent, Bianca Andreescu, was the new Rogers Cup champ by default. Williams was visibly upset by the development and began to cry as she rested on the sidelines.

Instead of celebrating her win, the 19-year-old Canadian walked over to the sidelines to give Williams words of encouragement.

“I’ve watched you your whole career,” Andreescu told Williams, according to HuffPost. “You’re a f**king beast.”

Andreescu also expressed empathy with Williams' back issues and acknowledged the difficulty of bowing out.

“I know how it is to pull out of tournaments and be injured, it’s not easy. This isn’t the way I expected to win, but you’re truly a champion,” she continued. “I’ve watched you play so many times, you are truly a champion on and off the court.”

The conversation ended with a hug and cheers from the crowd.

Williams was grateful for the pep talk and applauded Andreescu for her maturity, according to CBC.

"I was really sad and she made me feel a lot better," Williams said.

Andreescu earned respect from the tennis OG.

"I'm officially a fan," she said. "She's an old soul. She's only 19. She definitely doesn't seem like a 19-year-old. Her words on the court. Her game. Her attitude. Her actions."

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