“As I said in the article, I suck at goodbyes. But goodbye, Toronto,” she said in an on-field interview after the game.
Along with her older sister Venus, she helped transform the sport, regularly winning Grand Slam tournaments in both singles and doubles.
The 40-year-old has amassed 73 career singles titles, 23 doubles titles and two mixed doubles titles, including 39 Grand Slam titles – 23 singles titles, 14 doubles titles and two mixed doubles titles.
“It was a lot of emotions, obviously,” Williams told the crowd, speaking about the match.
“I love playing here, I’ve always loved playing here. I wish I could have played better but Belinda played so well today. It’s been an interesting 24 hours.
“Anyway, it is what it is. I was really happy to be here today and to play in front of you, thank you for the support.”
Fans gathered in the 12,500-seat stadium to watch Williams’ farewell and tournament organizers set up an outdoor viewing area for the first time ever which hosted 5,000 more.
Inside the stadium, the crowd held up signs saying “Thank you Serena”, “23 We will miss you” or simply “Queen”.
A farewell presentation ended with Williams accepting a gift from the Toronto Maple Leafs of the NHL and the Toronto Raptors of the NBA.
Williams is expected to play the Western & Southern Open next in Cincinnati, followed by the US Open which she says will be her last tournament.
Meanwhile, Coco Gauff defeated Wimbledon champion Elena Rybakina 6-4 6-7(8) 7-6(3) in a 2 hour, 49 minute marathon match on Wednesday to advance to the last 16.
Nick Kyrgios also qualified for the last 16 with an impressive 6-7(2) 6-4 6-2 victory over world No. 1 Daniil Medvedev.
CNN’s Ben Morse contributed to this report.
