* ATP Legg Mason Tennis Classic Right now, it seems no one can stop Juan Martin del Potro. He won his fourth straight tournament yesterday, beating Viktor Troicki, 6-3, 6-3, in Washington. The 19-year-old, ranked No. 19 in the world, has won his past 19 matches since June 25, when he was ousted from Wimbledon in the second round. "I feel confidence," del Potro said. "Because when I go to the court, and the other players know I beat 19 players in a row and nobody can beat me, it is good for me." The Argentine wasn't necessarily happy with his play yesterday, especially his service game - he had seven double faults to five aces - but his pinpoint ground strokes at big moments carried the match. "I didn't play too good," del Potro said. "But the important points, I played very good. That was the difference." The final game took longer to close out than del Potro would have liked. Serving with a 5-3 lead in the second set, del Potro wasted three match points. Then he held off three break-point opportunities before ending the match with an ace.