Defending champion Samantha Stosur has served up a treat to reach the last 16 of the US Open with a straight-sets disposal of American Varvara Lepchenko.

Stosur barely dropped a point on serve all match in an impressive 7-6 (7-5) 6-2 victory over the 31st seed.

Australia’s world No.7 will play either Chinese ninth seed Li Na or giantkilling British teenager Laura Robson in the fourth round on Sunday.

Stosur made a sloppy start, double-faulting on the opening point of the match, then hitting two horrible forehands to drop serve.

But Lepchenko was worse, coughing up two doubles and almost air-swinging one backhand to return the favour.

After finding her service groove, Stosur gradually began to control the match – but she was unable to convert any of four break-point chances and first set was decided in a tight tiebreaker.

Stosur clinched it after 59 minutes when Lepchenko hit wide.

Some forceful ball striking from the back earned Stosur a decisive break in the third game of the second set.

The titleholder consolidated with another love service game to charge ahead 3-1 and then broke the American’s serve – and spirit – once more to take a stranglehold on the match.

Yet another love hold advanced Stosur to 5-1 in the second and it was all over after one hour, 28 minutes when Lepchenko blasted a forehand return long on the Australian’s first match point.

Stosur’s victory was the 28-year-old’s 14th from her past 15 matches in New York.

Fellow Australian Lleyton Hewitt plays his second-round match against Gilles Muller later on Friday, while Bernard Tomic takes on retiring American star Andy Roddick in the feature night match.