Williams, whose last singles victory came at the 2021 French Open, had to battle hard to overcome Párrizas Díaz in close to two hours, despite what the straight-sets scoreline might suggest.
The 23-time grand slam champion dropped her serve just once in the match, but saved seven break points — including four when she trailed 3-4 in the second set — to hold off Párrizas Díaz, ranked No. 57 in the world.
It means Williams will face either Belinda Bencic or Tereza Martincova in the second round on Wednesday.
Playing in her first singles match in a year at Wimbledon in June, the 40-year-old lost to France’s Harmony Tan in an entertaining encounter and later hinted that she could play on home soil at the US Open, which begins August 29.
Asked on Monday what drives her to play at this late stage in her career, Williams said: “I guess there’s just a light at the end of the tunnel … Lately, that’s been it for me. I can’t wait to get to that light.”
“I love playing though, so it’s amazing. But, you know, I can’t do this forever,” she added. “Sometimes, you just want to try your best to enjoy the moments and do the best that you can.”
Against Párrizas Díaz, Williams showed signs of her trademark ball striking as she hit 32 winners, seven of which were aces.
According to Opta, she is the fourth woman since 2000 to win a WTA Tour match after turning 40, alongside Martina Navratilova, Kimiko Date Krumm and Williams’ sister, Venus.
The 42-year-old Venus was also in action in Toronto on Monday but lost 6-2 6-3 to Switzerland’s Jil Teichmann.