0732 GMT January 23, 2020
Nadal, who won his fourth title in Canada a year ago, needed almost two hours on Friday to overcome the Italian veteran who shocked him on clay at Monte Carlo in April, AFP reported.
"It was an up-and-down match, but a positive match for me because I played better and better as it went on," Nadal said.
"I lost the first set feeling that I was not playing badly. Things were just going too fast.
"Things happen quickly here because the court is fast. Sometimes in the first set, the player who is taking advantage has the adrenaline very high.
"It's difficult to quickly stop that momentum."
World number two Nadal was the only major seed to survive a quarterfinal cull after defeats for Dominic Thiem and Alexander Zverev.
Austrian second seed Thiem, coming off an emotional home victory in Kitzbuehel last week, hit the wall in a 6-3, 6-1 thrashing at the hands of Russia's eighth-seeded Daniil Medvedev.
Number three Zverev was unable to make an impression on Russian sixth seed Karen Khachanov, losing 6-3, 6-3 in 74 minutes.
Zverev suffered the first blemish on what had been a perfect 7-0 record in Montreal after winning the 2017 edition in his maiden visit to the city, which alternates with Toronto as host venue each year.