The French 15th seed defeated 10th-seeded Ukrainian Alexandr Dolgopolov 7-6 (11/9), 6-3 to advance to the title match of this US Open tuneup, where he'll face ninth-seeded Austrian veteran Jurgen Melzer, a 6-2, 4-6, 6-3 winner over sixth-seeded American Sam Querrey.
The win was the first for the 32-year-old Austrian over Querry after two defeats.
Melzer has lost to Monfils in four previous matches, but Monfils said the injuries that have hampered him in recent weeks make him the underdog.
"I'm trying get back into shape. I've worked hard this week and it's paying off well."
More From This Section
Monfils called for the trainer after just 11 minutes on court, having surrendered two breaks of serve to trail Dolgopolov 0-3.
But after a consultation and on-court treatment, he changed his attitude and got on with the job of mounting a comeback.
"I had thigh muscle pain which was going to my hip. The trainer told me I was just tight," Monfils said.
"I tried to keep it simple and it worked," added Monfils. "I needed to get my rhythm back.