Four-time champions Serena and Venus Williams withdrew from the women’s doubles competition at the Australian Open on Wednesday, officials said. Tennis Australia, which coordinates the withdrawals for the tournament, did not specify why the two were unable to play in the first round against 11th seeds Anabel Medina Garrigues of Spain and Yaroslava Shedova of Kazakhstan. Both Serena and Venus Williams are due to play their second-round singles matches on Thursday.