World no. 1 is main motivation: del Potro

Argentina's Juan Martin del Potro warhned that he was ready for an assault on the top of the world rankings after winning the Japan Open. looking down the list of tennis greats engraved on the trophy, he said, "Amazing names. when you win this kind of tournament, it means a lot. I believe in myself for sure. I'm working hard to improve. There is no bigger motivation than becoming number one in the world."