Williams' last singles success was at the 2021 French Open.

She will play either Olympic gold medallist Belinda Bencic or Tereza Martincova next.

Williams made her singles return at Wimbledon in June after a year out with injury after speculation she may retire.

When asked what drove her to continue playing, she 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, so it's amazing but, you know, I can't do this forever.

"Sometimes you just want to try your best to enjoy the moments and do the best that you can."

A three-time champion at the Toronto event, Williams was given warm and vocal support by the crowd in her first hard court match since the 2021 Australian Open.

She and Parrizas Diaz exchanged breaks in the early stages, before a backhand smash gave the 23-time major winner a 5-3 lead and chance to serve for the set.

Williams then saved a break point of her own, before finally taking the set on another smash at the net.

Although her serve and movement were not at their best, she produced some superb defence to save another break point and keep the second set on serve.

That continued into a 25-minute service game that saw nine deuces, with Williams somehow coming through to hold before breaking Parrizas Diaz in the next game.

After converting her first match point, Williams congratulated her 31-year-old opponent, telling her she "played really well" before acknowledging the crowd.

The Toronto competition is one of several warm-up events for the US Open, which begins on 29 August.

Wimbledon champion Elena Rybakina and Simona Halep also advanced on the opening day's play.