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.

Welsh cynghanedd group Carmarthen Bardic School is celebrating its 25th anniversary and hopes to publish a collection of poems written by past members.
It has helped train a number of National Eisteddfod winners including Mererid Hopwood, who tutors for the group, and Aneirin Karadog.
The lessons are held over a pint at the Carmarthen Quins Rugby Club.
Cynghanedd has bee present in Welsh language rhyming since the 6th Century, and developed into a complex set of poetic rules from about the 13th Century.
Geraint Roberts, one of the original members, said: "There's a very close correlation between the development of the Welsh language and cynghanedd.
"We set homework and that's a critical part of it - going over the homework, looking over mistakes and looking for ways of improving and developing."
One of its youngest members, 15-year-old Dafydd Llywelyn, said: "My forefathers learnt this craft and I want to keep the tradition alive."

In one case, reported by German broadcaster NDR, a shackled Moroccan man was allegedly forced to eat rotten pork off the floor at a police station.
Earlier, a policeman allegedly bragged on the WhatsApp messaging service that he had made an Afghan man choke until he "squealed like a pig".
The reports have shocked officials.
A police officer's home and workplace have been searched and investigators found a photo on his mobile phone showing a man being held down in a painful stress position.
Witnesses are being questioned over the alleged abuses, according to NDR.
The head of human rights group Pro Asyl, Guenter Burkhardt, said the allegations indicated "an appalling level of racism and inhumanity".
In one WhatsApp message, quoted by NDR, the policeman wrote: "I knocked him away. An Afghan. With an immigration ban. Stuck my finger up his nose. And he choked. It was funny. And dragged him in shackles through the (police) station. It was so great. Squealed like a pig. It was a gift from Allah."
An unnamed police source was quoted as saying "there was often screaming from the holding cells, and when it got too piercing they didn't look into it - they just shut the door, to stop the sound getting out".
The German government official in charge of integration, Aydan Ozoguz, called the allegations "shocking".
She said that if a policeman had actually tortured migrants then the police must "make it absolutely clear that they do not tolerate such inhumane behaviour in their ranks".
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.
[NEXT_CONCEPT]
A group is calling on former members to contribute poems for a new book.
[NEXT_CONCEPT]
Prosecutors in northern Germany are investigating alleged cases of police brutality towards migrants in the city of Hanover.