World number one Andy Roddick says doping is not a problem in tennis, even though Britain's Greg Rusedski and Mariano Puerta of Argentina have recently tested positive for banned drugs.

"I know the players who I play with are clean," Roddick told reporters as he prepared for the Kooyong Classic exhibition event which begins here on Wednesday.

"I see on a daily basis how hard people work. That's the way I approach it.

"It takes just a couple of people who maybe have done the wrong thing to put the focus there [on doping]," said American Roddick.

Rusedski admitted last week that he tested positive for the steroid nandrolone at a tournament in Indianapolis last July.

But the left-hander has denied taking performance-enhancing drugs and will carry on playing until a hearing in Montreal on February 9.

Rusedski said 47 players had shown elevated levels of nandrolone and argued that he had been singled out.

Organisers of next week's Australian Open have said that Rusedski will be welcome at the first Grand Slam of the season.

Last Wednesday, Puerta was banned for nine months, fined and stripped of ranking points after testing positive for clenbuterol during an ATP event in Chile last year.

Among the players joining Roddick at the Kooyong event will be Australian Open champion Andre Agassi and Wimbledon champion Roger Federer.