Defending champion Petra Kvitova, however, became the latest victim of Sorana Cirstea's fine run at the hardcourt event in Toronto.
World number one Williams broke Magdalena Rybarikova's serve six times as she eased past the Slovakian 6-1 6-1.
"I'm definitely happier with the return game today," the American said on www.wtatennis.com after ending the winning run of Rybarikova, who retained the Citi Open title in Washington last week.
"I could still use some improvement. She served pretty well, but I was pretty happy with the results from today."
The 16-time grand slam winner will tackle Agnieszka Radwanska in the last four after the Polish third seed overcame Sara Errani, the fifth seed from Italy, 7-6 (7/1) 7-5.
Cirstea, meanwhile, won seven consecutive games when trailing by a set and 4-5 en route to a 4-6 7-5 6-2 victory over sixth-seeded Czech Kvitova.
The win continued a brilliant week for the Romanian, who ousted ninth seed Caroline Wozniacki and 15th seed Jelena Jankovic in the previous two rounds.
She will next face Chinese fourth seed Li Na, who maintained her 100 per cent career record against Dominika Cibulkova by toppling the Slovakian 7-6 (7/1) 6-2.
"It was very tough to win the first set, but once I got it, I was thinking, 'Okay, one set in the pocket, so just keep hitting the ball'," Li said.
"In the second set I was just continuing what I was doing and not looking at what she was doing any more. I was feeling very good in the second set."