(Reuters) - Japan's third-seeded Kei Nishikori came from 2-5 down in the opening set to stun Swiss second seed Stan Wawrinka 7-6(6) 6-1 in the semi-finals of the Rogers Cup in Toronto on Saturday.

Wawrinka, a two-times grand slam champion who has won 14 ATP World Tour titles, became increasingly dispirited as he lost 11 of the last 13 games at the Aviva Centre after squandering four set points in that opening set.

Nishikori, serving at 40-0, sealed victory on the first of his three match points when an overhead smash prompted an unforced error from his opponent, ending an encounter that lasted just under an hour-and-a-half.

Nishikori will be seeking his first ATP Masters 1000 title after reaching the third final of his career in the elite events, and will next face either top seed Novak Djokovic or 10th seed Gael Monfils who were playing later on Saturday.

"I knew I would have to step it up a little bit in the second set the first couple of games," Nishikori told ESPN in a courtside interview after beating Wawrinka for only the second time in five meetings.

"And after that, I tried to be more aggressive and really concentrate because he was a little bit down and I knew if I had to let him, he's going to come back.

"I am very happy to be here in the final tomorrow but I really have to play my (best) tennis to beat either Novak or Monfils. He's been playing great."

Wawrinka made a confident start against Nishikori, breaking him in the fourth game of the match to lead 3-1 and then 5-2.

However, the Swiss failed to hold serve in the ninth, saved two break points in the 11th to edge ahead 6-5 but was unable to convert two set points in the 12th as the opening set went into a tiebreak.

Wawrinka again held an early advantage as he built a 6-4 lead in the tiebreak after Nishikori netted a forehand but the Swiss hit a backhand wide and then double faulted to squander two more set points.

The Japanese clinched the opening set after just over an hour on his first opportunity when Wawrinka hit a backhand long and raced to the finish line as he completely dominated the second.

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Andrew Both)