Rafael Nadal practices in Toronto, Canada on Tuesday. Photo: VCG

A nagging left foot injury forced Rafael Nadal out of the ATP Toronto Masters on Tuesday as top seed Daniil Medvedev rallied to reach the third round.

Nadal, a five-time champion in Canada, pulled the plug on his participation before his scheduled second-round opener with plans to return to Spain for treatment of the injury which kept him from Wimbledon and the Tokyo Olympics.

"I've had this issue for a couple of months," he said. "It is not a happy situation after all the success that I had here in Canada.

"I need to go back [to Mallorca] and try to find a way to be better again. With this pain, I'm not able to enjoy [playing].

"I believe that I am not able to compete at the level that I need because the foot doesn't allow me to move the way that I need," added Nadal, who was the second seed in the hard-court tournament. 

Top-seeded Russian Medvedev opened his week in the second round after a bye and staged a fightback to overhaul Kazakh Alexander Bublik 4-6, 6-3, 6-4.

The Russian came back a changed man after a second-set rain delay. Medvedev advanced with 27 winners and 28 unforced errors in the two-hour victory.

New world No.3 Stefanos Tsitsipas cured an extended case of the jitters with a runaway third set to ­finally overcome Ugo Humbert 6-3, 6-7 (13/15), 6-1.

With the win he avenged a pair of previous losses to Humbert, including in the third round of the Olympics.

The Greek wasted five match points in the second set while his French opponent held on in a 23-minute tiebreaker.

But after a speedy deciding set, Tsitsipas finally prevailed on his seventh match point, finishing with 37 winners including 16 aces.

Sixth seed Casper Ruud made a smooth transition to hard court ­after winning three consecutive claycourt titles, knocking out Marin Cilic 6-3, 3-6, 6-3.

The Norwegian, who has moved to 10th in the rankings, won his 36th match of the season.

Kei Nishikori, runner up in 2016, carried his Tokyo Olympic and Washington momentum into a 6-4, 6-7 (5/7), 6-2 first-round win over Miomir Kecmanovic.

Asia's leading player, who reached the quarterfinals at the Games and followed up by reaching the semifinals in Washington, broke the 59th-ranked Serb five times in their match lasting for just over two hours.

Olympic silver medalist Karen Khachanov defeated Britain's Cameron Norrie 6-4, 5-7, 6-4 with 10 aces, lining up a date with 15th-seeded fellow Russian Aslan Karatsev.

American John Isner, winner of a sixth Atlanta trophy earlier this month, pounded 19 aces in his 6-4, 6-1 thrashing of Spaniard Alejandro Davidovich Fokina.