World number one and defending champion Rafael Nadal has withdrawn from the AEGON Championships at Queen's and faces a battle to be fit for Wimbledon. The Spaniard, a surprise fourth-round casualty at the hands of Robin Soderling in the French Open, has been advised by doctors to rest because of ongoing knee problems. He said: ``I have been having some problems in the past months with my knees, that's no secret, that have not allowed me to always compete at 100%. ``I need to work with my team to recover well, work on my physical condition to be at my top form and get ready for the grass to play at Wimbledon. ``I hope I can be ready to compete by then.''