Argentine Mariano Zabaleta upset second seeded American James Blake 7-5, 7-6 on Saturday to advance to the final of the US Men's Clay Court Championships against Croatia's Ivo Karlovic.

The world number 156 handled the windy conditions well, edging the ninth-ranked player 9-7 in the second set tiebreak to reach his first final since 2004.

Earlier, the big-serving Karlovic advanced to his second final of the season with a 7-6, 6-2 victory over unseeded Spaniard Albert Montanes, seizing the momentum after winning the first-set tiebreaker 7-5.

Sunday's final will be the first meeting between the players.

Blake, 27, appeared to be in control of the match until Zabaleta broke his serve to go up 6-5 in the first set. The American then double-faulted to lose the second-set tiebreaker and the match.

Zabaleta said it was the first time he had even reached the semi-finals of a tournament since a knee injury in November 2005 sidelined him for eight months.

"I feel really happy and I enjoy this moment," he told reporters.

He added that strong winds and cool temperatures made conditions difficult, but he won because he was very focused on the important points.

Blake's third loss this season under blustery conditions left him feeling he had some work to do to improve that aspect of his game.

"It's pretty frustrating. I need to learn how to play better in bad weather," he said.

He blamed his double fault to end the match on a failure in concentration, but said Zabaleta deserved the win.

"Under these conditions, he played pretty well. He definitely earned it," Blake said.