(CNN) -- Defending champion, Samantha Stosur is safely through to the fourth round of the U.S. Open after a straight-sets victory over 31st seed Varvara Lepchenko on Friday.
The Australian had to fight hard against the American in the first set but had an easier time of it in the second eventually prevailing 7-6 (7/5) 6-2.
"As the match went on I got a little more comfortable and relaxed a little bit more. In the first set I felt like I served really well. That's how I was able to get into those games in the second set," Stosur said.
The seventh seed now faces British teenager Laura Robson, who continued her remarkable run with victory over ninth seed Li Na of China.
After seeing off three-time U.S. Open champion Kim Clijsters on Wednesday, Robson repeated the trick against Na on Friday, winning 6-4 6-7 (5-7) 6-2.
"I knew if I kept sticking with her and playing my shots it might pay off and it did," Robson said after her latest triumph.
"I had a few break points early on and I gave them up with average returning, so when I got a chance I knew I had to go for it," she added.
The 18-year-old's previous best at a grand slam was reaching the second round of Wimbledon in 2011, but she now is one match away from the last eight.
Seventh seed, Stosur admitted the match against Robson, who is currently ranked 89th in the world, will be tough.
"She's already had two very good wins and she's full of confidence. I'm going have to be ready to play my best game if I'm going to beat her," Stosur said.
"It can be a very tricky match. She's got nothing to lose ..." she added.
In other matches, Maria Sharapova thrashed America's Mallory Burdette 6-1 6-1 to advance to the last 16, while fifth seed Petra Kvitova from Czech Republic beat France's Pauline Parmentier 6-4 6-4.
Kvitova's win sets up a match against another French opponent, Marion Bartoli.
The 11th seed enjoyed a comfortable 6-2 6-4 victory over compatriot Kristina Mladenovic.
Sixth seed Angelique Kerber of Germany ended Venus Williams dreams of a third U.S. Open title with a 6-2 5-7 7-5 win over the American.