Laura Robson of Britain congratulates Samantha Stosur (R) of Australia after their women's singles match at the US Open tennis tournament in New York September 2, 2012.?[Photo/Agencies]

NEW YORK - Defending champion Sam Stosur ended the giant-killing run of 18-year-old Briton Laura Robson with a hard-fought 6-4 6-4 win in the fourth round of the US Open on Sunday.

The Australian came from a break down to win the first set and held off a late fight-back by Robson in the second to advance to the last eight after one hour, 38 minutes.

Robson saved eight match points as she came from 5-2 down to get back on serve but Stosur broke again to clinch victory and a match with top seed Victoria Azarenka.

"On the match points I probably had a few flashbacks to a few matches last year when I had match points and couldn't close it out," Stosur said.

"But I thought it was a good match and I played pretty well. Towards the end, I probably stepped it up another gear. It was a really good match and definitely one that I needed going to the quarters."

Robson had beaten Kim Clijsters and Li Na to make the fourth round and showed why as she crunched a few groundstrokes to break serve in the third game.

Stosur broke back immediately thanks to a few Robson mistakes but she was in trouble at 4-4 when the Briton had two chances to break.

But the Australian held serve and then broke to win the set, before storming ahead 5-2 in the second.

In a mammoth eighth game, Robson saved five match points to hold serve and then saved two more as she broke the defending champion to get back on serve.

Robson then saved an eighth match point as Stosur showed signs of nerves closing it out but the seventh seed then finally sealed it on the ninth attempt.

Vary Serve

Stosur said she had tried to vary her serve to keep the hard-hitting Robson off balance.

"I've been serving very well out in all my matches, but I really did want to try and mix up my serve today and not let her get grooved.

"She's a very good ball striker. You give a player like her the same shot over and over, eventually you're going to get burned."

Robson will rise from 89th to around 75th in the rankings after her run to the fourth round but said she had believed she could have gone further.

"I'm definitely disappointed," the former Wimbledon junior champion said. "I felt like I didn't play as well as I did in my last three matches. I just hit too many unforced errors.

"Sam is someone who sort of makes you feel like you're playing bad because she hits it with so much spin, so it's hard to get her serve back sometimes.

"But it's been a good tournament for me."

The $120,000 for reaching the fourth round is the biggest payday of Robson's career to date and said she intended to spend some of it on Monday.

"I'll go home and train for a week before I go out to Asia (for a series of tournaments)," she said. "But I think I've got tomorrow off to do some shopping."

Stosur has struggled to get the better of Azarenka and admitted she will need to play her best if she is to continue the defence of her title.

"You've got to give it everything you've got," she said. "I have not done well against her in the past. There's a few things I've learned over those last few matches.

"Hopefully I can put all those into play, have a better match and hopefully a very good one."