The win makes Ron, ranked 40 in the world, the first Dane to reach the quarter-finals of Roland Garros, where he meets Norway’s Casper Ruud.
The 19-year-old is a former French Open junior champion and, according to the ATP tour, is also the first man from Denmark since Jan Leslie in 1967 to reach the quarter-finals of a Grand Slam tournament.
“I’m feeling great now,” Ron said in his on-court interview after Monday’s game. “I was very nervous at the end but the fans were great for me throughout the match, the whole tournament.”
He won a tight first set against Tsitsipas last year in just over an hour before the Greek tied the match 4-3 thanks to a break in the second set.
Ron regained the advantage with a break midway through the third set to advance 5-2 in the fourth set to finish the win.
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Along with Spaniard Carlos Alcaraz, Ron is the second teenager to reach the quarter-finals of the men’s singles draw at Roland Garros this year. This is the first time since 1994 that two boys have reached the quarter-finals of a major.
It caps an amazing year for Ron, who took his first win over a top-five player when he defeated Alexander Zverev at the Munich Open in April.
He will play Ruud on Wednesday after the 23-year-old beat Poland’s Hubert Hurkacz 6-2, 6-3, 3-6, 6-3.
Meanwhile, Tsitsipas has become the highest-ranked player – and first in the world’s top eight – to be knocked out of the men’s lottery at this year’s French Open.