Nadal, who won this event for the fourth time last year, battled through the wind and broke Pella's service four times in the triumph.
Playing in his first action since losing to Roger Federer at Wimbledon last month, the 33-year-old Spaniard enjoyed an easier outing than his opening match against England’s Daniel Evans on Wednesday when he rallied from a tiebreak deficit.
Nadal will now face seventh seed Fabio Fognini after the Italian beat France's Adrian Mannarino 6-2, 7-5.
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"He's playing great," Nadal said of Fognini after pulling ahead of Federer with his 379th ATP Masters 1000 win.
"I saw him a little bit yesterday. He played a great match. He's having a great season, one of the best of his career, if not the best.
“We are in quarterfinals of a Masters 1000. All opponents are tough. For me, tomorrow is an opportunity to go on court against one of the best players, try to play a good match. So let's see if I am able to.”
In earlier action, Austrian second seed Dominic Thiem beat Croatian Marin Cilic 7-6(7) 6-4 to reach the Rogers Cup quarter-finals.
Thiem, who before this week had never won a match in five attempts in Canada, had his hands full against 14th-seeded Cilic but did well to save all nine break points he faced while relying on a steady stream of groundstroke winners.
The Austrian, who arrived in Montreal fresh off a claycourt title in Kitzbuhel where he was the top seed, said his triumph on home soil helped ease the transition to hardcourt tennis.
"The last three, four years I didn't play worse than this year, but I just came with low confidence and I came little bit sad to Canada because I didn't play good in my own home tournament," said Thiem.
"This year everything is different. I won this title at home. I made one of my biggest dreams come true. Of course, I'm so confident coming here. That's the difference. That's also made the transition pretty easy."
Up next for Thiem will be Russian eighth seed Daniil Medvedev, a 6-3 6-3 winner over Chile's Cristian Garin.
In other action, sixth-seeded Russian Karen Khachanov rallied to beat 19-year-old Canadian birthday boy Felix Auger-Aliassime 6-7(7) 7-5 6-3 to reach the quarter-finals.
Third seed Alexander Zverev of Germany will face Khachanov in the quarters after a 7-5 5-7 7-6(5) win over Georgia's Nikoloz Basilashvili.
Spain’s Roberto Bautista advanced past Frenchman Richard Gasquet 7-5 7-5 while France’s Gael Monfils defeated Hubert Hurkacz 6-4 6-0. (Reporting by Frank Pingue in Toronto and Jahmal Corner in Los Angeles; editing by Ed Osmond/Amlan Chakraborty)
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