MONTREAL - John Isner's big serve proved too much for Vasek Pospisil, the last Canadian left in the singles play at the men's Rogers Cup.

The six-foot-10 Isner pounded 15 aces en route to a 7-6 (1), 4-6, 6-3 victory over Pospisil on Wednesday night at the US$4.1 million hardcourt event.

Isner, the 16th seed, posted his second win in as many weeks over Pospisil, a Vernon, B.C. native who fell to the American last week in Washington, D.C. Isner is 10-1 in his last 11 matches overall.

"He served great," said Pospisil. "Even from the baseline he was playing quite well.

"He didn't have any hesitations. That's just a sign of somebody that's won a lot of matches."

Pospisil, ranked 45th, reached the semifinals the last time the Rogers Cup was held in Montreal in 2013, but this one ended in the second round despite the support of the packed centre court grandstands at Uniprix Stadium.

"Even walking off the court after the defeat, which was tough, for sure, it was still an incredible crowd. The atmosphere was great," he said.

The first set started and ended poorly for Pospisil. He double faulted on break point in the opening game, but evened the set at 2-2 with a remarkable service return on break point. It went to a tiebreaker, where Isner aced all four of his serves and Pospisil double faulted on set point.

The Canadian opened the second set with a service break and held his own to tie the match.

Isner, who got 72 per cent of his first serves in, finally broke for 4-2 in the third, when Pospisil double faulted to give the American the advantage and then hit a groundstroke long on break point.

Pospisil had a break point in the next game, but Isner powered his first serve in to go to deuce and ended it with two aces.

Earlier, Kei Nishikori followed his tournament victory in Washington, D.C. last week with a 6-3, 6-3 win over Pablo Andujar. The fourth-seeded Nishikori posted his ATP Tour-leading 27th hardcourt win of the year.

David Goffin, seeded 13th, will be Nishikori's opponent in the round of 16 on Thursday after a 6-4, 6-4 win over Sam Querrey.

Rafael Nadal, coming off a win on clay at Hamburg two weeks ago, got his tournament started with a 7-6 (4), 6-3 win over Sergiy Stakhovsky. He next faces Mikhail Youzny, who upset ninth-seeded Gilles Simon 6-1, 6-4.

Nadal, a winner of 14 career grand slam events, has slipped to ninth in the world after an up-and-down year. But back to full health, Nadal hopes to climb back to the top.

"I won in Hamburg a couple weeks ago and this was an important victory here," he said. "Six consecutive (match) victories, that didn't happen very often during the season.

"That's a positive moment for me."

American Jack Sock downed 14th-seeded Grigor Dimitrov 5-7, 7-6 (5), 7-5 to advance to a meeting with top-seeded Novak Djokovic.

Gilles Muller upset Gael Monfils, the 15th seed, 6-3, 3-6, 7-6 (4) and next gets second seeded Andy Murray, who beat Tommy Robredo 6-4, 7-5.

Jeremy Chardy downed Leonardo Mayer 6-4, 7-6 (4) to set up a third round meeting with six-foot-11 Ivo Karlovic, the 36-year-old who ousted Canada's Milos Raonic on Tuesday night.

A pair of qualifiers got the to third round as Donald Young knocked off fifth seeded Tomas Berdych 7-6 (5), 6-3 and Ernests Gulbis beat Lukas Rosol 6-2, 5-7, 6-3.

Defending champion Jo-Wilfried Tsonga, seeded 10th, won a 5-7, 7-6 (6), 7-5 squeaker over Roberto Bautista Agut.