Monfils, the Frenchman ranked No. 43 in the world, overcame early struggles to beat 38th-ranked Alexandr Dolgopolov of the Ukraine 7-6 (11), 6-3 in the first semifinal at the Wake Forest Tennis Center.
"It's very good for my confidence," said Monfils, who will play in his third tournament final of the season and his first since Nice, France, in mid-May.
"I'm not so happy with the way I played. I could have played better. I have to play better and be more sure of things."
The 32nd-ranked Melzer, from Austria, then pulled off the biggest upset in the night match, beating American Sam Querrey -- at No. 29, the highest ranked player in the semifinals, and the crowd favorite -- 6-2, 4-6, 6-3.
"I've been serving well, and my ground strokes have been getting better from match to match," said Melzer, who also is playing in his third tournament final this season and his first since winning an ATP Challenger Tour event in Dallas in March.
"After losing in the first round of my last two tournaments, if somebody would have told me I could be in the finals, I would've signed it."
Monfils and Melzer each have four ATP tournament titles. Monfils holds a 4-0 lifetime record against Melzer, including a first-round victory in Munich in early May.
"Every title means a lot," Melzer said. "I've lost all the previous matches to him, so it will be a nice matchup tomorrow. Hopefully, I can pull it off, which would also be very good leading into the U.S. Open."
Monfils -- ranked No. 7 in 2011 -- lost serve twice early and went down 4-0 in the first set to Dolgopolov. The Frenchman even received treatment on his left abdominal muscles and hip during the side exchange after the third game of the set.
"I had a little pain before the match, and even during the match," said Monfils, who missed a month of action dealing with ankle and biceps injuries before playing in the final hard-court tournament before next week's U.S. Open.
"It was a bit better (after the trainer) worked on it, but I still felt it during the first set," he said. "It's not a tear, but it's very tight. I just have to take care of it, and I should be fine."
After the injury break, Monfils broke Dolgopolov's serve twice to force the tiebreaker and win the first set. He then added two more service breaks in the second set.
"It wasn't my best finish," said Dolgopolov, a Ukrainian who was playing in his first semifinal this season. "I was in the game, but every game I didn't finish out those important points. Game by game, he came back.
"I was angry at myself -- I had too many chances. When you have that in your mind, after having played three matches (in the tournament), it's tough to make yourself go out and forget about it."
Melzer -- ranked as high as No. 8 in 2011 -- broke Querrey's serve five times, and kept him off-balance for most of the match with his ground strokes and net game.
Still, Querrey fought back after losing the first set, getting a pair of service breaks en route to winning the second set. He then held serve to open the third set and grab the early advantage.
"I felt like I played really well for 1½ sets," said Melzer, who lost two previous matches against Querrey. "Then I kind of lost it a little bit, got a little too passive on my side and forgot about the game plan. I was lucky in the beginning of the third (set) to not get broken again. But after that, I started playing well again."
Querrey nearly pulled off another service break to open the third set, putting Melzer down 15-40 in the second game. But Melzer fought back to hold serve, then broke Querrey's serve the next game to take the advantage.
"I didn't feel my greatest out there," Querrey said, who lost in the semifinals for the second straight year. "I played a little sloppy in the first set, then that break in the second (set) helped me regain all the momentum. If I could've stuck that other break in the third set, maybe it could've been different."