(CNN) -- Reigning Olympic champion Rafael Nadal has been forced to withdraw from the London 2012 Olympics due to ongoing knee problems.
Nadal, gold medallist four years ago in Beijing admitted he was "not in the right physical condition to compete" and that he had to "think about Spanish sport."
The world number two, who was due to carry Spain's flag at the opening ceremony, had already taken two weeks off in the buildup to the Olympics to recover from tendinitis in his knee
"I've waited until the final moment of my preparation and my training, but I cannot do it," Nadal said in a statement released to the Spanish press on Thursday.
"Today is one of the saddest days in my career. Carrying the flag for Spain at the opening ceremony was set to be one of the biggest honors, one of the most special moments for me.
"For this reason I hope you can understand how difficult it has been for me to take this decision. One of my teammates who is better prepared than me can take my place and compete to the best of his ability."
The Spanish Tennis Federation announced that world number 30 Feliciano Lopez would take Nadal's spot in the singles competition. Marc Lopez, who has won doubles titles with Nadal on the ATP tour, will take his place in the doubles competition.
The Spanish Olympic Federation have chosen basketball star Pau Gasol as the new flag bearer.
Former world number one Nadal recorded his 11th grand slam win at June's French Open, but crashed out of Wimbledon in the second round to unfancied Czech player Lukas Rosol.
Nadal had only two weeks to make the transition from the clay courts of Roland Garros to Wimbledon's grass courts, but in future he will have longer to adapt.
Wimbledon organizers announced on Thursday that from 2015 there will be an extra week between the two slams.
"The best interests of tennis will be served by allowing the players more time to recuperate and to adjust from the clay of Roland Garros to the grass at Wimbledon," said All England Club chairman Philip Brook.
"We think most players welcome the prospect of a longer grass court season and spending more time on the softer surface of grass.
"In making this change from 2015 we recognize that there will be some important consequences for the overall tennis calendar, and enough time needs to be given to allow us all to plan accordingly."