World number one Williams, just days after bagging her fourth title of the year at Stanford, then joined her with a 6-0 6-2 thrashing of 2011 Flushing Meadows champion Stosur in a touch under an hour.
"It's been a few weeks since I've been in that competitive atmosphere," said Sharapova, who has not played since her fourth round loss at Wimbledon. "It's always quite different, no matter how much you train, you try to prepare for that.
"Once you step on the court, you feel a little bit more from the crowd, the energy. You get inspired by everything. But you're ultimately a bit rusty."
The French Open champion was far from sharp in her return as the promising young Spaniard took advantage of several errors by the Russian to grab the opening set.
But Sharapova, a three-time winner this season, was able to convert on the big points to stay on track for a first ever title on the Canadian hard courts.
"I didn't feel good, I made a lot of errors in the first set," said Sharapova. "When you're able to finish stronger than your start, that's always a positive because you give yourself an opportunity to keep playing in the tournament.
"When you're in that position, there's always another chance to work in another match to improve and get better."
Sixth seeded German Angelique Kerber, who has four runner-up finishes this season, had no trouble beating Caroline Garcia easing past the Frenchwoman 6-4 6-1.
Kerber has had a solid start to her North American hard court campaign after reaching the final in Stanford last week but in four visits to Canada has never advanced past the third round.
Eleventh seed Caroline Wozniacki of Denmark, the 2010 Canadian champion, thrashed Czech Klara Koukalova 6-1 6-2 while qualifier Heather Watson provided an early upset when the Briton shocked 10th seed and Australian Open finalist Dominika Cibulkova of Slovakia 6-2 6-7 (3) 7-6 (5).
Ninth seeded Serb Ana Ivanovic was also sent packing in the second round, losing to American qualifier Coco Vandeweghe in three sets.
(Editing by Frank Pingue/Greg Stutchbury)
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