Serena Williams came from a set and a break down to beat Flavia Pennetta at the Rogers Cup in Toronto, in her first match of the US swing.

Williams, preparing for a historic Grand Slam bid when she fronts up for the US Open starting on August 31, was broken five times in her first six service games on Tuesday, but ran out a 2-6 6-3 6-0 victor.

Pennetta had only once previously taken a set off Williams - in their first career meeting in Miami in 2008 - but the Italian had little trouble reeling off four consecutive breaks of the world number one's serve to claim the opener.

A former world number 10, Pennetta was well on her way to a significant upset when she broke again for a 2-1 lead in the second set - but that was where Williams' woes ended.

Williams affected back-to-back breaks to turn a deficit into a convincing lead, and lost just seven points on serve in her final six service games to avoid a shock result.

The 21-time major winner is through to the third round.

In first-round action, top-10 players Carla Suarez Navarro and Karolina Pliskova were surprise losers at the WTA Premier hard-court event - but Eugenie Bouchard was the biggest disappointment for the locals.

Bouchard fought gamely, but was ultimately humbled by Belinda Bencic - the Swiss winning 6-0 5-7 6-2.

The Canadian saved a match point in the 10th game of the second set with trademark aggressive groundstrokes and forced a decider, but it was to no avail in the decider - with 48 unforced errors blighting her performance.

Suarez Navarro, the ninth seed, was edged by Alize Cornet 6-3 6-7 (2-7) 6-4, while 10th seed Pliskova - the newest entrant to the tour's top-10 at number eight - was beaten by qualifier Mirjana Lucic-Baroni 3-6 7-6 (7-5) 6-2.

Thirteenth seed Angelique Kerber was a rampant 6-0 6-1 winner over Misaki Doi, while Sara Errani - seeded 15th - was eventually a comfortable 5-7 6-1 6-0 victor over Kristina Mladenovic.

Joining Lucic-Baroni as other qualifiers to progress were Carina Witthoeft, Olga Govortsova, Polona Hercog, Lesia Tsurenko and Monica Puig.

Witthoeft defeated Coco Vandeweghe 6-3 3-6 6-1, Govortsova saw off Irina-Camelia Begu 6-3 7-6 (7-4), Hercog overcame Alison van Uytvanck 6-4 1-6 6-3, Tsurenko defeated fellow qualifier Yanina Wickmayer 6-3 7-6 (7-3) - as did Puig against Mariana Duque-Marino 6-4 4-6 7-5.

Roberta Vinci took to fellow Italian Karin Knapp 6-0 6-0, losing just 16 points against a player ranked 18 spots higher than her - and it was a similar story between Daria Gavrilova and Sam Stosur.

Gavrilova, still a Russia representative despite being considered an Australian resident at grand slams, ousted Australia's number one Stosur 6-4 6-4.

Jelena Jankovic avenged her loss in the Bogota final last year to Caroline Garcia, earning a 7-6 (7-4) 6-2 win, while Barbora Strycova progressed after a 6-2 6-4 victory over Varvara Lepchenko.

Local hope Carol Zhao, a wildcard, was no match for Madison Brengle in a 6-1 6-1 rout, but lucky loser Julia Goerges earned a second-round berth after edging Zarina Diyas 2-6 6-1 7-5.

Alison Riske won a three-hour struggle over 12th seed Timea Bacsinszky 3-6 7-6 (7-4) 7-6 (7-3), while Victoria Azarenka had less stress in her 6-1 6-4 triumph over Elina Svitolina.

Andrea Petkovic, the 16th seed, knocked out Canadian wildcard Francoise Abanda 3-6 6-4 6-2, Heather Watson thrashed Irina Falconi 6-1 6-2, and Dominika Cibulkova downed Sloane Stephens 6-3 6-4.