US Open: Laura Robson admits sadness as Kim Clijsters retires

Laura Robson feels "sad" to have been responsible for beating Kim Clijsters in her final singles appearance.

Robson beat the former world number one 7-6 (7-4) 7-6 (7-5) to book a third-round US Open tie against Li Na.

Clijsters was hoping to mark her final singles event before retirement with a fifth major title.

"It's sad that it was her last match because I feel like she is still one of the best players in the world," said the 18-year-old Briton.

"She is someone I looked up to since I joined the senior tour. She has brought so much to female tennis, so it's tough she is going to retire. I wish her all the best."

Robson put her surprise win down to her added agility on the court.

"I've always been working on my movement. It's something I can still improve a lot. I'm putting a lot of work into it," she said.

"I'm getting more confident. I feel I can get to one extra ball every time and put more pressure on my opponent."

Clijsters was chasing her fourth title in New York, where she last lost in 2003.

But the British number three overpowered her Belgian opponent in two hours and six minutes at Arthur Ashe Stadium.

Since winning the Wimbledon girls' title at the age of 14 in 2008, Robson has grown to 5ft 11in tall.

Her agility has been questioned by a number of pundits but Robson, who will now face Chinese ninth seed Li, says she had been working with Andy Murray's fitness coach Jez Green to improve her game.

"Almost every day I'm doing some speed work," said Robson, who partnered Murray to Olympic silver in the mixed doubles event at Wimbledon earlier this month.

"I've had some tough matches against some top players where it's been a tough first set and I just haven't been able to keep up the level throughout a whole match.

"It's something that I've been working on, it's something that's getting better with more experience so it's great to be able to pull through."

Clijsters described the British teenager as one to watch in the future.

"I knew before I stepped out on court that I would have to play well. I've practised with her in Wimbledon. I said to my team she hits the ball so clean," said the 29-year-old.

"Physically if she gets better, even more explosive, then I think she's going to be a great player, and someone that I'll be keeping an eye on."

Billie Jean King, winner of 12 Grand Slam singles titles, said beating Clijsters would boost Robson's confidence.

"Laura is a promising player and having a win over Kim Clijsters is going to help her self-confidence and I think that's what she needs the most," the American told BBC Radio 5 live.

"Laura really needs to believe in herself and her own abilities and beating Clijsters is going to help her go one step further in her career."

Murray, seeded third in the men's draw, said his mixed doubles partner had played "some unbelievable tennis" on the main showcourt.

"She handled the situation well in a match that was big, everyone knew this could be Kim's last match," said Murray, who later reached the third round with a comfortable straight sets victory over Croatia's Ivan Dodig. "Laura's going to be very good but she needs time."