Australian Open day nine - the heat gets to defending champion Djokovic

Novak Djokovic plays a forehand during his quarter-final against Andy Roddick Photograph: Jason O'Brien/Action Images

Roddick plays a backhand return from the shadows Photograph: Quinn Rooney/Getty Images

A dejected Djokovic, silhouetted, walks back to his chair. The Serb retired in the fourth set while behind at 6-7, 6-4, 6-2, 2-1 – a victim of the searing Melbourne heat Photograph: Mark Kolbe/Getty Images

A spectator takes refuge from the sun under a parasol during a women's doubles match Photograph: Mick Tsikas/Reuters

Ai Sugiyama, left, celebrates with partner Daniela Hantuchova after winning a point during their doubles match against Cara Black and Liezel Huber. Sugiyama and Hantuchova won the match 6-7, 6-3, 7-6, (12-10) Photograph: Torsten Blackwood/AFP/Getty Images

Venus, left, and her sister Serena Williams are beseiged by spectators wanting their autographs before their women's doubles match against Peng Shuai and Hsieh Su-Wei Photograph: William West/AFP/Getty Images

Venus and Serena both try to return the ball during their 6-2, 4-6, 6-3 victory over Peng Shuai and Hsieh Su-Wei Photograph: William West/AFP/Getty Images

Vera Zvonareva watches the ball as she plays a forehand return during her crushing 6-3, 6-0 win over Marion Bartoli Photograph: Paul Crock/AFP/Getty Images

Bartoli of France plays a forehand in her quarter-final defeat to Zvonareva Photograph: Ezra Shaw/Getty Images

Dinara Safina celebrates winning a point during her closely fought quarter-final match with Jelena Dokic which the No3 seed won 4-6 6-4 4-6 Photograph: Petar Kujundzic/Reuters

Roger Federer watches the flight of the ball as he prepares to play a backhand return in his straight sets (6-3, 6-0, 6-0) quarter final victory against Juan Martin Del Potro Photograph: Oliver Weiken/EPA