World number two Victoria Azarenka has pulled out of Wimbledon after failing to overcome the knee injury she suffered on Monday.The two-time semi-finalist fell heavily during her 6-1 6-2 first round win over Maria Joao Koehler of Portugal.Earlier Steve Darcis, who beat Rafael Nadal in the first round, pulled out with a shoulder problem.Both John Isner and Radek Stepanek were also forced to retire in the early stages of their second round matches. More to follow.