LONDON (AFP) - Venus Williams and Elena Dementieva hardly broke sweat in Monday's sizzling Wimbledon heat as they cruised into the quarter-finals.

Defending champion Williams won the first set against former world number one Ana Ivanovic, who then withdrew with a leg injury, as temperatures hit 31 degrees at the All England Club.

Five-time champion Williams is bidding to become the first woman to win a hat-trick of Wimbledon singles' titles since Steffi Graf claimed three in a row from 1991 to 1993.

Third seed Williams cruised through the first set 6-1 in half an hour, after which Serbia's Ivanovic had her left thigh strapped up.

The 2008 French Open champion won the first game of the second set, but the pain was too much and she had no choice but to pull out, leaving Court One in a stream of tears.

"I felt really sad for her actually. She was really upset," Williams said.

"This is Wimbledon. It's the last place you want to have an injury that you can't overcome. So I'm wishing her a lot of luck in her recovery.

"She didn't have a lot of opportunities in that first set - and not because she wasn't playing well; I think I was just on top of the ball," the Californian added.

Fourth seed Dementieva, a semi-finalist last year, beat fellow Russian Elena Vesnina 6-1, 6-3 in 70 minutes on Court 2.

Vesnina fended off a set point and took the sixth game on a deuce but there was no stopping the Russian number two on her serve and Dementieva wrapped up the first set inside 29 minutes.

Vesnina, the world 37, put up a better fight of it in the second set, holding Dementieva at 2-2 before losing serve.

Dementieva, making her 11th Wimbledon appearance, now takes on either France's Virginie Razzano or Italy's Francesca Schiavone for a place in the semi-finals.

Meanwhile, Polish 11th seed Agnieszka Radwanska, a quarter-finalist last year, also went through in straight sets, against American teenager Melanie Oudin.

World number 124 Oudin, who had to qualify for the main draw, showed plenty of fight but the Wimbledon debutante was eventually overcome 6-4, 7-5.

Radwanska, who knocked out China's Peng Shuai and Li Na in the previous rounds, won the second set with three match points to spare to book a quarter-final berth.

The Pole now faces Venus Williams in the last eight.

Current world number one Dinara Safina, who has yet to win a Grand Slam title, faces 2006 Wimbledon champion Amelie Mauresmo on Centre Court.

The winner of that match will face either Denmark's ninth seed Caroline Wozniacki or Germany's Sabine Lisicki.

Elsewhere, two-time Wimbledon champion Serena Williams, the second seed, plays Slovakia's Daniela Hantuchova and Belarusian Victoria Azarenka takes on Russia's Nadia Petrova.