Wozniacki reached her first Grand Slam final in New York last year
World number two Caroline Wozniacki was made to work for a quarter-final place before eventually seeing off Flavia Pennetta at the Rogers Cup in Montreal.
The Dane came through 4-6 6-3 6-1 after getting a pep talk from her coach and father Piotr late in the second set.
Kim Clijsters looks in good shape for her imminent US Open title defence after racking up her seventh straight win 6-2 6-1 against Kaia Kanepi.
Defending champion Elena Dementieva lost 7-6 (7-3) 6-4 to Zheng Jie.
And former world number one Dinara Safina's mini revival came to an end with a 6-4 6-3 defeat by French Open champion Francesca Schiavone.
The Italian goes on to face Wozniacki, who was thankful for the intervention of her father in her third-round match.
"He just told me, 'you're playing great but she is too, hang in there and keep your concentration up and don't think you've won the set because she can come back'," said the top seed.
After needing three sets to beat Bethanie Mattek-Sands on Wednesday, fifth seed Clijsters wasted little time in sweeping past Kanepi for her 13th consecutive win on hard courts, having picked up the title in Cincinnati last week.
"It's always nice to win a tournament and gradually feel your level improve," said Clijsters.
"When you're down and able to fight back and win matches like last night it's something that is extremely important."
In other matches, 11th seed Svetlana Kuznetsova beat seventh seed Agnieszka Radwanska 6-4 1-6 6-3 and there were also wins for eighth seed Vera Zvonareva, 10th seed Victoria Azarenka and 17th seed Marion Bartoli.