Stefanos Tsitsipas knows what it takes to topple iconic champions.

The 21-year-old Greek has beaten each of the Big 3—Rafael Nadal, Novak Djokovic and Roger Federer—this season. Tsitsipas stunned Federer at the Australian Open, upset Nadal on his home soil in Madrid and surprised Djokovic in the Shanghai quarterfinals last month.

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So which of the Big 3 poses the toughest test?

Tsitsipas says it's world No. 1 Nadal.

"I would go for most players' answer: Rafa Nadal," Tsitsipas told the media in London. "That's what everybody says. I totally agree with that."

Why Nadal?

Tsitsipas says the Spaniard's ferocious fighting spirit, point-to-point intensity and comprehensive court coverage make him the beast of the Big 3.

"Well, first of all, the intensity, his fighting spirit, his speed, movement on the court," Tsitsipas said. "He covers much court when you play against him. So it's not very easy to hit winners or play aggressive shots that are going to lead to more defensive balls.

"I just think mostly his fighting spirit and that he believes in himself so much when he's out on the court. He has one of the best forehands in the game, too."

Photo credit: Mark Peterson/Corleve