Juan Martin Del Potro, coming off an epic Wimbledon semi-final loss to world number one Novak Djokovic, says he is ready to close the gap on the world's top tennis players. The seventh-ranked Argentine is the men's top seed for this week's US$1.76 million ATP and WTA Washington Open, his first event since losing the longest semi-final in Wimbledon history 7-5, 4-6, 7-6 (7-2), 6-7 (6-8), 6-3 after four hours and 43 minutes. It was a very good experience for me even in the loss. I played a good match with the number one in the world in the semi-finals at Wimbledon. Many good things to take "It was a very good experience for me even in the loss," Del Potro said. "I played a good match with the number one in the world in the semi-finals at Wimbledon. Many good things to take. "It was a really long battle for both of us. We played an incredible match for about five hours with high intensity. After that I needed a week to recover myself. And Novak was really tired in the final. Everyone saw that." Djokovic lost in the final to Britain's Andy Murray but remains atop the rankings and that duo together with Rafael Nadal and Roger Federer remain the men to catch in Del Potro's eyes. "That's my biggest challenge for the next year," Del Potro said. "The top guys are so good. The top four have a big difference to the rest of the players. I have to do well in the big tournaments to get close, but that's not easy. "They are much better than the rest of the players. Me and the rest of the players, we have three or four good tournaments and we get injuries or some bad tournaments. I know the way to beat the top guys, but it's not easy." Del Potro, 24, is the only player since the 2005 Australian Open outside of the star foursome to win a grand slam singles crown thanks to his 2009 US Open final victory over Federer. Del Potro resists the idea that because players are more friendly with each other than in past eras the on-court rivalry is less intense. "Of course at the end of the [Wimbledon] match I would like to kill Novak for beating me. But I couldn't," Del Potro said. "Novak is a friend of mine. That match looked like a war but we're still normal people. Del Potro, the 2008 and 2009 Washington winner, will open against either Australian Lleyton Hewitt or American Ryan Harrison later today after a first-round bye. He has practiced for two weeks after having doctors check his left knee following Wimbledon, where he aggravated an injury. "I'm OK. I'm healthy," Del Potro said. "I twisted my knee but nothing dangerous. I had the MRI and studies in Argentina and doctors say everything is good."