Nadal won last year in Toronto for his fourth title in the event. He has 82 singles victories, winning the French Open in June for his second title of the year.
Monfils, seeded 16th, outlasted 10th-seeded Roberto Bautista Agut of Spain 6-4, 3-6, 7-6 (2) in a match suspended Friday night because of lightning. Monfils finally finished off Agut after a rain delay Saturday, but battled a sore left ankle that forced him to pull out of the semifinal.
Monfils consulted with his coach about the decision to withdraw.
“He said, ‘You can’t take the risk,'” Monfil said. “I had retired in Wimbledon. I came back. I played this week. I had played a huge match. It was risky to play another match because I could risk hurting myself for real.”