Serena Williams admits she was "shocked" when she heard fellow American Andre Agassi had taken drugs while on the ATP tour and subsequently lied to tour officials.

Agassi last week revealed in his autobiography he had tested positive for the highly-addictive drug, crystal methamphetamine, in 1997 and then duped the Association of Tennis Professionals into believing he had taken it by accident to escape a ban.

Williams said: "I was shocked. I was like 'wow,' but he was always a different character and so I wasn't really that shocked because he was kind of a wild child.

"Pete Sampras was more like an arrow and he (Agassi) was a punk rocker kind of guy.

"It was unfortunate for tennis because players in the last six, seven, eight years have been tested all the time. The anti-doping laws are very strict and severe to the point where they recommend you not to take some vitamins because they could be cross-contaminated.

"It is disappointing for tennis because tennis is one of the cleanest sports."

Williams, who claimed the number one spot for the fifth time in her career when beating sister Venus in Doha on Sunday, revealed she was tested for drugs on a regular basis.

Serena added: "If a girl played me and I ended up losing a big match and she was taking a performance-enhancing drug that would be very disappointing, but I've never been in a situation like that.

"It's not very good being in the sport and being clean and working so hard and always getting tested and then having someone come into the sport and do that (take enhancing drugs).

"I think what he (Agassi) was doing would have decreased his performance.

"The best thing is to say no to drugs, that's my policy. I make that plain and clear."