HOUSTON  - Fifth seeded John Isner claimed his sixth career title and first on clay by beating Nicolas Almagro in straight sets in the final of the US Clay Court Championships on Sunday.The hard-serving American blasted seven aces en route to a 6-3, 7-5 win over the Spanish clay court specialist at River Oaks Country Club in Houston. Isner, who was the runner-up in this event last year, went one better this time, being broken just once in the 81-minute match."I've always known I could play well on clay," said Isner. "This week is a little surprising, as Monday was the first day I hit a ball on clay since September. "I knew it was going to be a tough adjustment and that I had to find a way to get through my first match. My second match was really close. I felt I played better each and every round. I played well yesterday and even better today."Even though all 12 of Almagro's career titles have come on clay, he couldn't make it 13 on Sunday, failing to overcome four double faults and three service breaks. "He did a really good job today," Almagro said of Isner. "He played aggressive. "He served like normal, but today was tough for me. I want to congratulate him, because he's a really good player and a good person and I think he did a good job this week."Isner, who finished second to Juan Monaco last year, avenged that loss with a three-set win over the Argentine in the semi-finals on Saturday. The world number 23 Isner had lost both previous matches to Almagro, including a five-set loss in the Davis Cup semi-finals in 2012. Almagro was seeking to win his first title in 2013 and, like Isner, was appearing in his first final of the year. He fell to a 17-7 season record and a 12-7 mark in Tour finals. Isner earned $82,000 in first place prize money.