MELBOURNE: Andy Roddick stormed into the Australian Open semi-finals on Tuesday as exhausted Novak Djokovic withdrew from his quarter-final but it was full steam ahead for ice-cool Vera Zvonareva.Despondent defending champion Novak Djokovic pulled out with cramp while trailing 7-6 (7/3), 4-6, 2-6, 1-2.The Serbian world number three looked a forlorn figure as he conceded to the American seventh seed after becoming more and more hampered on a scorching hot day.Roddick, who has now reached the semi-finals in Melbourne four times, was playing high class tennis at the time and may well have progressed regardless.In an Open that has produced plenty of surprises, Djokovic became the fourth player to pull out of the quarter-finals either injured or ill, following in the footsteps of Zheng Jie, Victoria Azarenka and Gael Monfils.He blamed his retirement on cramps and the heat, with the mercury reaching 35 C (95 F).Roddick's next opponent will be either 13-time Grand Slam champion Roger Federer, who has often been the American's nemesis at major tournaments, or exciting young Argentine Juan Martin del Potro.Like many others here over the past nine days, France's Marion Bartoli also felt the effects of the searing heat in her match against Russian seventh seed Zvonareva, bending double and ++++ing for breath.She started strongly but rapidly wilted, crashing 6-3, 6-0 in a one-sided drubbing to a player who is in her first Grand Slam semi-final in 25 attempts.Zvonareva next plays either third seed Dinara Safina or Australian wildcard Jelena Dokic, who is on the comeback trail after years of depression following the antics of her infamous father Damir.