John Isner is developing such a strong affection for claycourt tennis that he is emerging as a dangerous outsider for the French Open. Isner steered the United States into the Davis Cup semi-finals at the weekend, beating Gilles Simon and then Jo-Wilfried Tsonga in four sets on the famous clay courts of the Monte Carlo Country Club. France had been warned of the 26-year-old's ability after Isner defeated Roger Federer, also on clay, in February's first round 5-0 Davis Cup whitewash of Switzerland. Just for good measure, Isner has also beaten world number one Novak Djokovic this year in the semi-finals of the Indian Wells Masters albeit on a more familiar hard court. "I like playing on clay," said Isner. "I played three of my best matches this year, my best wins have come on clay. It's a surface I enjoy playing on and a surface I feel I can do very well on." His big, power game has been at home on Europe's notoriously slow clay courts -- in his 6-3, 7-6 (7/4), 5-7, 6-3 win over Tsonga, he fired down 16 aces and unleashed 56 winners. France and Switzerland may have been stunned by 6ft 9in (2.06m) Isner's claycourt prowess, but not USA captain Jim Courier, who knows his way around the gruelling, slow surface, winning back-to-back French Opens in 1991 and 1992. "I am not surprised by the way John has played. We were hoping he could be at this kind of level and he was. He was exceptional," said Courier. "He is continuing to progress. He was a lot stronger than he was against Switzerland. My players are comfortable on all surfaces, they understand clay courts." The USA, 32-time Davis Cup champions, face a third road trip in this year's tournament when they travel to face defending champions Spain in September's semi-finals. They have the unenviable task of ending Spain's 23-tie unbeaten run on home soil. But world number 11 Isner, who pushed Rafael Nadal to five sets in a first round defeat at the 2011 French Open, relishes the challenges ahead, believing that his game measures up to all of his rivals. "With that serve and with that forehand he's got the ability to be a Top 10 player," said Djokovic after his defeat at Indian Wells. "He had that already a couple years back. It's just a matter of really trusting your instincts and your strokes and your quality. I think it all came together for him now." Isner, briefly at number 10 in March, shared Djokovic's belief. "I want to go even higher. I really do believe that I have that in me," he said.