Latvia's Ernests Gulbis survived a few anxious moments in the final set but held his nerve to upset top-seed Mardy Fish 5-7, 6-4, 6-4 and win the Los Angeles International on Sunday.
Serving for the match at 5-2, Gulbis was broken by American Fish and then trailed 30-40 on serve at 5-4 before sealing victory.
The Latvian set up match point with a delicate forehand drop shot before clinching his second ATP title with a rasping forehand winner down the line.
"It means a lot to me. It's my second title," Gulbis said. "It gives me a huge boost in confidence."
Gulbis, who had knocked out second seed Juan Martin del Potro of Argentina in the quarter-finals, pumped his right fist in celebration after becoming the first unseeded champion in Los Angeles since Radek Stepanek in 2007.
The triumph was Gulbis's first victory over a top 10 opponent this season. Fish fell to 6-13 in ATP World Tour Finals.
"I've played a lot of matches the past month and half, so he came in a little bit fresher than me. I think that played a part," Fish said.
"But credit to him. He played well when he needed to. He got those two breaks early [in the third set], clearly which was big. I almost got him in the end... it wasn't meant to be."