World No. 3 Rafael Nadal has admitted he is concerned he will not be fully fit for the defence of his Olympic title at London 2012.Nadal, who has been chosen to carry the Spanish flag at the opening ceremony, suffered a shock second-round defeat to Lukas Rosol at Wimbledon and has since been undergoing rehabilitation on a persistent knee problem.He will return to the All England Club later this month as he bids to become the first man to defend his singles gold medal.“I am doing everything possible to recover,” Nadal said. “After Roland Garros I had a difficult time, but the excitement is at its maximum, I am working as hard as I can and hopefully I will be at 100 per cent.“I am the one who is most worried. I hope that things improve and my intention is to arrive in London well prepared and recuperated. It is for this that I work every day. Carrying the flag will the something unforgettable and I hope that Spain continue on the path of success it has been on for the past few years.”Nadal, who won Wimbledon in 2008 and 2010, insists his surprise defeat to Rosol has not dented his confidence on grass.“Opponents intimidate everyone, you can’t hide when you are competing against the best,” he said. “You go onto court knowing that you can win or lose. You have to accept both things. I have already had years of competing and I know that there are bad moments and defeats.”