LONDON (AFP) - Defending champion Venus Williams took the express route into the quarter-finals at Wimbledon as Ana Ivanovic was forced to quit Monday's fourth round match with a thigh injury.

Williams was already well on course for the last eight after taking the first set 6-1 before Ivanovic called for treatment on her left leg during the opening game of the second set.

The Serb, seeded 13th, served out the game but was unable to carry on and left the court in tears.

Venus will next face Poland's Agnieszka Radwanska, the 11th seed, as she strives to keep alive her bid 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 five-time Wimbledon champion has yet to drop a set in four rounds this year and her winning streak at the All England Club has now reached 18 matches.

Even more remarkably, she has won 30 consecutive sets here since Japan's Akiko Morigami took the second set of their third round match in 2007.

Although it was Ivanovic who eventually succumbed to injury, the day had started with concerns over Venus after her father Richard Williams revealed over the weekend that his daughter's knee injury was restricting her movement.

The American once again wore a heavy strapping on her left knee but Williams senior had no need to worry.

Ivanovic, the 2008 French Open champion, wasted three break points in the first game and was punished when she had the temerity to test Venus's movement with a drop shot in the next game.

The American's answer was emphatic as she raced to the net and left Ivanovic floundering with a cross-court winner to reach break point.

She converted the break and was 4-0 up in just 19 minutes as Ivanovic wilted in the sweltering conditions on Court One.

With the first set wrapped up in double quick time, Venus was about to go for the kill when Ivanovic called for treatment on her thigh injury.

The Serb had her thigh strapped and won the first game of the second set, but the pain was too much and she had no choice but to pull out.