Marco Cecchinato beat former champion and 12-time major victor Novak Djokovic 6-3, 7-6, 1-6, 7-6 to become the first Italian man to reach a Grand Slam semi-final in 40 years.
Either way, Tuesday's outcome was stunning.
The last-eight match of Roland Garros between Zverev and Thiem will start at 2 pm local time, 5:30 pm IST, 1 pm BST.
"A hard one to swallow", Djokovic acknowledged during a brief news conference, in which he delivered clipped answers and said he might not play during the upcoming grass-court season.
The last Italian man to take part in a major semi-final was Corrado Barazzutti at the 1978 French Open.
"This is the best moment of my life", Cecchinato said.
The Austrian is aiming to reach the French Open semi-finals for the third year running, after he lost in the last four to the eventual victor in both 2016 (Novak Djokovic) and 2017 (Rafael Nadal). "I have practiced with him many times in Monte-Carlo", Djokovic said. At 8-7, Cecchinato ended a 20-stroke exchange with a swinging volley victor. My heart was beating 1,000 miles per hour. He is a 25-year-old from Sicily who once was handed a match-fixing suspension that later was thrown out on appeal.
Told in an on-court interview that he wasn't dreaming, Cecchinato responded: "Are you sure?"
He has declined to discuss the case in Paris.
Zverev prevails in third consecutive five-setter to set up Thiem clash, a long awaited matchup since the draw was announced Alexander Zverev has reached his first Slam quarterfinal the hard way, beating Karen Khachanov 4-6 7-6 (4) 2-6 6-3 6-3 for his third straight comeback win in five sets. If you do, leave this match alone from a pre-match betting point of view, but if you feel that Zverev will be as fresh as a daisy, then Zverev at [2.56] would be a viable consideration.
Thiem has already beaten 10-time champion Rafa Nadal three times on clay and either he, or Zverev, could be big threat to the Spaniard if they meet in the final.
"I'm just taking it one match at a time, and right now, considering I played nearly four hours in my last match, I felt pretty good physically today", said Djokovic.
He might well need to up that schedule considering his quarterfinal opponent is No. 7 Dominic Thiem of Austria, who is one of the fittest players on the men's circuit and has reached the past two semifinals at Roland Garros.
Djokovic, who missed the last half of 2017 with right elbow trouble and had surgery in February, is clearly not at the height of his power.
So it was a quarter-final greeted with much intrigue and excitement as both men looked to move a step closer to a potential first Grand Slam title.
By the end, though, points were tight and taught, one even lasting 30 strokes.
Of the last 10 Masters, only three have been won by the old guard, two for Nadal, one for Federer.
"I had to push. So it's going to be an wonderful challenge for me". "I am still aiming to play in the coming weeks, but I want to be 100 percent when I do return".