Melbourne  - Spanish tennis star Rafael Nadal on Monday stood up for US swimmer Michael Phelps, who apologized after being photographed with a pipe usually used for marijuana consumption.

"It does not portray the best possible image, but everyone can have a slip-up. In the end sportsmen are just not machines," tennis world number one Nadal said. "Everyone is free to do whatever they want."

A photograph of Phelps was published in an English tabloid on the weekend that showed the swimmer using a long glass pipe. The photograph was allegedly taken at a party in South Carolina.

Phelps admitted that he was the person in the photograph, but did not admit to smoking marijuana, nor did he say whether there was any marijuana in the pipe.

Although not considered a performance enhancing drug, marijuana is on the list of prohibited substances and several sportsmen and women have been banned for use of marijuana.

Phelps, who has won 14 Olympic gold medals during his ongoing career - six in Athens 2004 and eight in Beijing 2008, - said in a statement that his behaviour "was regrettable and demonstrated bad judgment."

"He has been on holiday for some time," Nadal said of fellow- Olympic champion Phelps. "He has attained all his goals and he is taking a sabbatical leave and he is totally free to be able to do it. He will have plenty of time to get back and prepare for the coming Games."

Nadal and Phelps professed mutual admiration during the Beijing Olympics and even met briefly and had photographs taken together. (dpa)