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Laura Robson believes tactical advice from new coach Zeljko Krajan has played a significant role in her surprise run to the fourth round of the US Open.

Robson, 18, in August having never been past the second round of a Grand Slam.

"The work we've done has been very specific in terms of tactical stuff," said Robson, who plays defending champion Sam Stosur on Sunday.

"In the last few weeks I have gained a bit of confidence."

Robson victory over Kim Clijsters.

Three-time champion Clijsters had no answer to her opponent's power and variety as the Briton secure the biggest win of her career.

Robson followed it up with another shock win, beating ninth seed and 2011 French Open champion Li Na in three sets.

"I think the level has always been there," said the world number 89. "I have always thought that I can play with the top girls.

"Whenever I've practised with Caroline [Wozniacki] or Maria [Sharapova], I've always felt that the level was there.

"It was just taking that on to the match court and keeping the level up for the whole match."

"In the last few matches it's noticeable that instead of just trying to hit a winner off a tough shot like I have done in the past, I'm trying to make a percentage shot back. That makes all the difference really."

Robson, who won the mixed doubles silver medal with Andy Murray at the London Olympics, feels improved fitness has also played a part in her rise.

"It's been massive," she explained. "It makes so much of a difference mentally. I just feel like I'm healthy.

"This is my third match and I'm feeling fine, I'm feeling fit. I'm really ready for the next match, but in the past, after the first round here, I had needed a lot of physio work.

"I have an infected toe at the moment, but it's not bothering me and I'm feeling fine."

Of seventh seed Stosur, Robson added: "I have had a fairly tough draw, haven't I? You have to beat who is in front of you. That's what I've managed to do so far.

"I play Stosur now, who is defending champ. That's going to be really tough. I've never played her before so I'm going to just work hard."

The experienced Australian is not taking Robson lightly.

"It can be a very tricky match," said the 28-year-old. "She's probably got nothing to lose.

"She's had two of the best wins of her career, and she's starting to maybe live up to some of the potential that people have talked about from when she won Junior Wimbledon when she was 14.

"You can't go into it lightly. You've got to be aware that she's going to come out swinging and have that confidence behind her.

"But you've got to stick to your own guns, play the way that you need to play. When it comes down to the crunch, back yourself."