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 kind of character and 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 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.”