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Williams, a former world number one and 13-time Grand Slam champion, defeated Czech Lucie Hradecka 6-3, 7-6 (7/5).
The American booked a second-round clash with Australian Samantha Stosur - the woman Williams beat on Sunday in the championship match at Toronto.
Stosur, the 10th seed, advanced with a 6-3, 6-1 victory over qualifier Eleni Daniilidou of Greece.
"Sam is playing great," Williams said. "We had a really tough match in Toronto, and it was a big win for me.
Williams returned in June from an 11-month absence due to injury and illness, beginning with a cut foot that required surgery shortly after her 2010 Wimbledon triumph and including dangerous blood clots in her lungs.
She has improved her world ranking from 175th in the world to 31, winning titles in consecutive starts at Stanford and Toronto.
Her 12-match winning streak matches Kim Clijsters and Victoria Azarenka for the longest on the WTA tour this year.
Hradecka, who had battled back from 1-5 down in the second set to take a 6-5 lead en route to the tiebreaker, finally double-faulted on match point.
Williams admitted she was frustrated that she couldn't close out the match when she led 5-1.
"I was definitely up and I didn't close it out, and she started playing different and better and running a lot," she said. "I was struggling at that point."
She said fatigue might have been a factor, but she'd rather cope with that than with too little time on court.
"I would rather have that than fresh legs from not winning," she said.
