The 19-year-old, ranked No. 19 in the world, has won his last 19 matches since June 25, when he was bounced from Wimbledon in the second round.
Serving with a 5-3 lead in the second set, del Potro used two aces to go up 40-15. But on two match points, he first put a backhand, then a forehand, into the net. He quickly earned another match point chance, and had yet another backhand go into the net.
Del Potro then held off three break point opportunities for Troicki before ending the match with a 134-mile-an-hour ace.
Troicki seemed to have a sore back during the match. After losing his serve to go down 4-3 in the first set, Troicki took an injury timeout and laid on the court while his lower back was worked on. The next game, Troicki was unable to return del Potro's serves, losing four straight points and often grabbing his lower back in pain.
The soreness again appeared to bother him as he had two double faults in the final game of the first set.
Del Potro also looked shaky serving at times - he had seven double-faults to five aces - but his pinpoint groundstrokes carried the match.
All four of del Potro's career tournament wins have come during this streak. He won clay tournaments in Stuttgart, Germany and Kitzbuhel, Austria in July. He won his first hard-court title last week, beating Andy Roddick in the final of the Countrywide Classic in Los Angeles.
OCHOA WINS FOURTH STRAIGHT TITLE
Del Potro, Haas advance to Legg Mason quarters
