Swiss second seed Wawrinka's game went away during the tiebreaker, allowing Nishikori to move into his second Masters 1000 final of 2016, yesterday.
The third seed lost to Novak Djokovic at the final in Miami.
"Both guys are playing great tennis right now, especially Monfils," Nishikori said.
"He's won Washington.
"It's going to be interesting. I haven't beaten Novak for a long time on hard court. In Miami he kind of dominated the game. So I hope I can get revenge this time."
Also Read
"It's just great to be into another Masters final, I'm looking forward to my opportunity," the winner said.
Nishikori was caught on the back foot in the early stages as Wawrinka secured an early break of serve, only to lose it in the ninth game of the opening set.
With the battle heading into a tiebreaker, the double Grand Slam champion Swiss took a 3-1 lead. But that slowly slipped away as his 37 unforced errors mounted.
The second seed was all about the Asian ace as Wawrinka's game evaporated.
Nishikori closed it out on the first of three match points.
"I just tried to concentrate in the tiebreaker," Nishikori said.
"I played a point at a time. I took confidence into the second set, tried to be aggressive on returns and stood farther back than usual."
Wawrinka now stands 3-2 in the series while Nishikori continues to gain confidence as he plays for the first time since Wimbledon. The Japanese player now has 40 wins this season.
"I should have played better. That's it," Wawrinka said.
"But to make semi-finals in a Masters 1000 here on hard court where I never really played well on fast hard court - it's a good tournament. There are many things that I can be happy with.