LONDON (AFP) - Venus and Serena Williams hardly broke sweat in the sizzling Wimbledon heat Monday as they cruised into the quarter-finals. Defending champion Venus easily won the first set against former world number one Ana Ivanovic, who retired hurt in floods of tears, as temperatures shot past 30 degrees at the All England Club. Serena strolled through against Daniela Hantuchova, while Elena Dementieva also had a comfortable ride into the last eight against Elena Vesnina. Five-time champion Venus 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. The third seed 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. "This is Wimbledon. It's the last place you want to have an injury that you can't overcome," said Venus, who faces Poland's Agnieszka Radwanska for a place in the semi-finals. "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 American added. Ivanovic said: "I just felt a sharp pain on my inner thigh and I couldn't step on my leg ever since." Serena, the second seed, beat Hantuchova 6-3, 6-1 in 56 minutes on Court 2. The two-time champion easily brushed aside the unseeded Slovak, who has been suffering with a cold and now tackles Belarusian Victoria Azarenka. "I feel like I definitely need to step it up, play better, really start playing some great tennis, or go home. And I don't want to go home, so I feel like I'm just getting more serious," Serena said. Fourth seed Dementieva, a semi-finalist last year, beat fellow Russian blonde Vesnina, the world number 37, 6-1, 6-3 in 70 minutes. "I was able to break her a couple of times and just went into a good lead in the beginning of the match and stayed very focused until the end," Dementieva said. "Because of the weather conditions, I was trying to make it as quick as possible." Dementieva, making her 11th Wimbledon appearance, now takes on Italy's 43-ranked Francesca Schiavone for a place in the semi-finals. Schiavone beat France's number 26 seed Virginie Razzano 6-2, 7-6 (7/1). Azarenka, the eighth seed, had a tough time of it to beat 10th-seeded Russian Nadia Petrova. Azarenka won the first set 7-6 (7/5) on a tie break, after which Petrova called a medical time-out and was treated with ice on her legs. The Russian won the second set 6-2 but lost the final set 6-3. Current world number one Dinara Safina, who has yet to win a Grand Slam title, faces 2006 Wimbledon champion Amelie Mauresmo on Centre Court in Monday's final ladies' fourth round match. Mauresmo won the first set but Safina was 4-1 up in the second when rain stopped play and the Centre Court roof was deployed mid-match for the first time. The winner will face Germany's 41-ranked Sabine Lisicki who beat Denmark's ninth seed Caroline Wozniacki 6-4, 6-4. The teenagers had a brief clash before the final game, walking into each other as they crossed at the net. Lisicki knocked out French Open champion Svetlana Kuznetsova in the previous round. Meanwhile, 11th seed 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.