Williams prevailed in 90 minutes and showed none of the uncharacteristic struggles that dogged her during last week's semi-final loss to Swiss teenager Belinda Bencic in Toronto.
The world number one, who will aim for a rare sweep of the four grand slam events at the Aug. 31-Sept. 13 U.S. Open, broke Pironkova's serve once in each set and then sealed victory in style with her 15th ace of the second round match.
"There wasn't anything really tactical in particular for me today, I always just try to play my game and be consistent and stay focused," Williams, who improved to 44-2 in 2015, said in a courtside interview.
The American will meet Karin Knapp in the third round after the Italian advanced with a 6-4 6-1 win over Russia's Daria Gavrilova.
Williams is skipping next week's Connecticut Open in order to prepare for the U.S. Open, where she will look to earn her fifth consecutive grand slam singles title and fourth of 2015.
"There's definitely a lot of pressure... I would like to win the Open, probably more than anyone else," she said.
"Whether I do or not, I'm going to try win it this year or next year and just keep going for that. So, it's not the end of the world."
KVITOVA, BOUCHARD BEATEN
In the evening session, Azarenka continued her dominance of Wozniacki when she eased past the fifth seed 6-0 6-4.
Though ranked well below her opponent due to an extended injury layoff, it was the fifth consecutive straight sets victory for the world number 21 from Belarus over the Dane.
Wozniacki had her chances in the second set but frittered away 10 of 11 break points, while Azarenka converted two of her four chances.
The Dane, who has fallen at the first hurdle against three different opponents the past three weeks, has been granted a wild card entry into next week's Connecticut Open in an effort to find some form before heading to the U.S. Open.
Fourth seed Petra Kvitova also bowed out, the Czech beaten 7-5 4-6 6-2 by Caroline Garcia, of France, leaving the women's event with just two of the top five seeds still standing.
Garcia will meet 14th seed Elina Svitolina in the next round after the Ukrainian edged out the slumping Eugenie Bouchard 7-6 (6) 7-5 as the Canadian's woeful season continued.
In earlier action, Romanian third seed Simona Halep beat Frenchwoman Kristina Mladenovic 7-5 5-7 6-4 while Czech seventh seed Lucie Safarova advanced with a 6-4 7-5 win over American wild card Coco Vandeweghe.
(Reporting by Tim Wharnsby in Toronto; Editing by John O'Brien)
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