Reilly Opelka and John Isner will meet in the US Men's Clay Court Championship final in Houston, after they claimed semi-final wins on Saturday.

Opelka kept up his strong early-season form with an imposing 6-3 7-5 victory over Nick Kyrgios in the first semi-final, getting the job done in just one hour and nine minutes.

Opelka slung down 18 aces to 11 from Kyrgios and faced only one break point across the contest.

The 24-year-old Opelka won a title in Dallas in February and followed up with a run to the Delray Beach final, with a third trophy match of the season now awaiting him.

The American world number 18 said he admired the "intangibles" from Kyrgios that "you can't teach."

"He's got one of the best serves in the world and when he turns it on he can come up with some shots that not many guys outside the top 10 can," Opelka said in his on-court interview.

Opelka enjoyed the high-tempo match with Kyrgios, who typically wastes no time between points.

"I prefer to play at that speed rather than, for example, Rafa [Nadal] where it's 50-55 seconds between points," Opelka said.

"It's a totally different speed here and I thought I did a good job managing the whole tempo of the match. I put a lot of returns into play and I came up big with my second serve."

He will face 2013 Houston champion Isner, who came from a set down to defeat defending champion Cristian Garin 4-6 6-3 6-4.

The 36-year-old's serve improved as the match went on and ultimately propelled him to victory, facing four break points in the opening set, one in the second and none in the decider.

Isner also served 17 aces on his way to the final, and won 80 per cent of points on his first serve.

"It's never easy when you have to come back from one set down," he said. "My serve certainly helped me out a lot. I didn't have to hit too many balls there in the end, and I didn't want to.

"I'm so happy to be in the final again here. At my age you never really know when you can get back to a final of a tournament like this."