Two-time Roland Garros finalist Dominic Thiem says there are two main reasons why he believes tennis is clean.

While some former Top 10 players, including Maria Sharapova, Richard Gasquet, Martina Hingis and Marin Cilic have each failed a drug test—Sharapova served a 15-month ban for meldonium, Gasquet and Hingis both tested positive for cocaine and Cilic was given a nine-month ban after the stimulant nikethamide was found in his system—Thiem asserts top players are clean these days.

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The world No. 5 says frequent drug-testing is the primary reason he believes players aren't doping.

"First of all, because myself, I get tested so often," Thiem told the media after his straight-sets loss to Mattero Berrettini in the ATP Finals. "And then also I see all the other top guys getting tested that often. That's the first thing why I'm sure [players are not doping]."

The man who made history as the first Austrian to reach the semifinals of the ATP Finals this week says the second reason he believes the sport is clean: performance ehanching drugs don't enhance ball-control skills.

Thiem asserts while PED users can gain strength and stamina, there is no drug that can sharpen eye-hand coordination and racquet skills. The Indian Wells champion believes world-class hand skills cannot be improved through PED use.

"The second thing is because we are still playing a ball game, and I mean, it matters how fit you are, how fast you're running, but still, the most important is how you hit the ball and how you play the game and that you cannot effect with any medication," Thiem said. "If you could, it would be very nice, but there is no such substance existing."

Photo credit: Christopher Levy