Haas, who turns 35 in April, will meet Milos Raonic after both men came through semi-finals in straight sets.
"At my age, you're always looking for another title," Haas said. "Hopefully I can play well again tomorrow.
"You can always improve your game. You might start thinking that all these younger guys have passed you by, but you can help your all-around game. That's what you have to keep thinking."
Isner said: "Normally I can serve better than how I did. That's what decided the match. He played a lot better than I did. He didn't miss too many serves. He was very good."
Top seed Raonic made short work of American Sam Querrey, booming 12 aces in a 6-4 6-2 victory.
"He's the only guy in the field who has been over 140 (mph) on his serve consistently all week," Querrey said of Raonic, who has not lose a service game all tournament.
"Even if you can get your racket on it, you might not get it clean. You try to get the ball in play and go from there. I wasn't even close to getting to it."