Defending champion Sam Stosur ended British teen Laura Robson's run at the U.S. Open in straight sets Sunday.

The seventh-seeded Australian finally clinched victory on her ninth match point to win 6-4, 6-4 and reach the quarterfinals. Robson had defeated former Grand Slam winners in her last two matches, but the streak ended against Stosur, who has yet to drop a set here this year.

The 18-year-old Robson, who had upset Kim Clijsters and Li Na, double-faulted to hand Stosur the first set.

Leading 5-2 in the second, Stosur had five match points on Robson's serve, but the 89th-ranked teenager saved all of them. Stosur then failed to serve out the victory, wasting two more match points. But she broke Robson in the next game to win in 1 hour, 38 minutes.