Murray won 6-3 6-0 in 65 minutes to set up a final against world number one Novak Djokovic on Sunday (20:00 BST).
The result means the Scot will overtake Swiss Roger Federer in Monday's new ATP rankings and return to number two for the first time since 2013.
"It's good to get back close to the top," Murray said.
The US Open gets under way on 31 August, and it was following the same tournament last year that Murray lost his place in the top 10 after losing to Djokovic in the quarter-finals in New York.
"It's nice to get back especially after everything I went through last year with the hip surgery and dropping out of the top 10. A lot of questions were being asked about how my game was physically.
"I'll keep working hard and try to get that one spot higher."
Djokovic beat unseeded Frenchman Jeremy Chardy 6-4 6-4 in the first semi-final.
The Serb improved his record against Chardy to 10-0 with victory in one hour and 20 minutes, despite requiring several bouts of treatment on a sore right elbow.
"It comes and goes, now is not very pleasant," Djokvoic said of the problem. "Every day is a new day. I try to bury the pain, play with the pain, it is part of what we do."
Murray, 28, powered away from Nishikori, converting six break points and winning the last eight games in a row, including 12 of the last 13 points.
He now faces another chance to end a losing streak against Djokovic that stretches across eight matches back to the 2013 Wimbledon final.
Britain's Andy Murray swept past Kei Nishikori to reach the final of the Rogers Cup in Montreal and guarantee a return to second in the world rankings.