"I'm expecting another five-set match", Zverev said. Twice already he had made the semis at Roland Garros, and twice made the final of the Madrid Masters.
Thiem showed no mercy as he continued to dominate, racing into a 4-0 lead in a flash in the third set by mixing up his game expertly.
Play was suspended due to fading light, with Russian Daria Kasatkina leading 7-6 (5) 3-3 against second-seeded Caroline Wozniacki.
After that, Novak Djokovic will play against Marco Cecchinato, who is having an fantastic year so far having won his first ATP title in Budapest in April and having reached a Grand Slam quarter-final for the first time in his career. Nadal and Federer have returned from injury and surgery respectively to dominate the scene since early a year ago, topping the ranks, winning the last five Majors between them, with Nadal favourite to win his 11th French Open this week. "And I think that's good enough". "It will be a pleasure to play against Novak".
The world No. 3 keeps surprising his audiences, but not as much as Italy's Marco Cecchinato, who shocked eighth seed David Goffin 7-5, 4-6, 6-0, 6-3. His tour-level career record was 4-23 before this season.
The first men's quarter-final at this year's French Open was the latest in what is promising to be a glimpse of the future of men's tennis.
Keys next faces unseeded Yulia Putintseva of Kazakhstan, who beat 26th-seeded Czech Barbora Strycova 6-4, 6-3.
We saw a magnificent fightback from Diego Schwartzman yesterday, with the Argentine fighting back from 1-6 2-6 down (and saving match points) to beat Kevin Anderson and move into the quarter-finals.
"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.
My model agreed with this line, and in short, I'd be utterly amazed if the former world number one was unable to extinguish the Italian's hopes of making his first Grand Slam semi-final, in what is his first Grand Slam quarter-final.
While Thiem was a bundle of energy, Zverev looked somewhat tired - as well as being troubled by his left leg - and he was a break down at 2-1 in the second after making a mess of a backhand slice.
"At the end I was nervous".
Three games later, the Austrian served out the opener in 41 minutes, finishing with an ace. "So hopefully next time I will win it..."
After looking down and out for much of the third set, second seed Zverev was sacked up by a code violation for being coached from the stands by his father Alexander Sr midway through the fourth set and he vented his anger by instantly breaking for a 4-2 lead.