Isner beat Spain's Feliciano Lopez 6-7 7-6 6-3 , while Monaco defeated qualifier American Michael Russell 5-7 6-1 6-4.

Isner is 7-0 on clay this year, while Monaco is 10-2 on the surface, including a victory in Vina del Mar in February.

The 6ft9in Isner has won 44 consecutive service games in Houston and sent down 13 aces against world number 15 Lopez.

The win ensures Isner will become the United States' top-ranked player at number nine in the world when the new list is announced, overtaking Mardy Fish, who lost in the second round here on Thursday.

"Coming out of college, I never would've imagined (it)," said Isner. "As I continued to improve, I knew that this was potentially attainable for me. Very thrilled to be the top-ranked American. Now, I have a my highest ranking ever, too."

The 26-year-old Isner is 20-6 this year, including victories over Roger Federer and Novak Djokovic.

Sunday's final will be Monaco's second straight match against an American opponent after he beat Russell in the other semi-final.

Monaco said: "It was difficult to get a rhythm. They were tough conditions, but I'm happy to win the match and be in the final."