World number two Roger Federer has called for the introduction of biological passports in tennis similar to those used in cycling to detect possible doping.
"A blood passport will be necessary as some substances can't be discovered right now but might in the future, and that risk of discovery can chase cheaters away," the 31-year-old Swiss said on the opening day of the World Indoor Tournament in Rotterdam.
"But there also should be more blood tests and out of competition controls in tennis," he added.
According to figures on the International Tennis Federation website (www.itftennis.com), sport's governing body carried out only 21 out-of-competition blood tests in the professional game in 2011.
Cycling's governing body the UCI carried out more than 3,314 out-of-competition blood tests in the same year.
The UCI introduced biological passports in 2008 to track any blood changes in riders against an original profile which could mean they had taken illegal substances.
"I didn't get tested on blood after the Australian Open and I told the responsible people over there that it was a big surprise for me," said Federer, who lost to Briton Andy Murray in the semi-finals.
"But there also will be more funding needed to make all the tests possible and the Grand Slam tournaments should help to finance that as it is in their best interest to keep the sport clean and credible."
Federer said he had the impression that his sport was clean.
"The past years we had something like one case a year and often it had to do with unintentional mistakes made by players," he said. "But even then they should not make those mistakes and know the rules and live by them."
Defending champion Federer begins his quest for a third Rotterdam title agains Slovenian Grega Zemlja on Wednesday.