But the greater power of Keys paid dividends in the breaker, as the American finally took the opener on her fourth set point with her 18th victor.
The German had come through three consecutive five-set matches but his efforts appeared to take a toll, with a scan set to discover the extent of the damage. The elder man, the super-fit Thiem with a one-handed backhand of match-winning power, led their head-to-head 4-2, including their only French Open meeting when Zverev was still a teenager.
Play was suspended due to fading light, with Russian Daria Kasatkina leading 7-6 (5) 3-3 against second-seeded Caroline Wozniacki.
It was hard to discern which was less likely: that 12-time major champion Novak Djokovic would falter in his French Open quarterfinal or that Marco Cecchinato, who never won a Grand Slam match until last week and once faced a possible ban for losing on goal, would rise to the occasion.
"Well, I'm young, so I might as well stay on court, get some practice and entertain you guys", smiled Zverev, who is bidding to become the first German man to win the French Open since 1937.
Thiem was knocked out by Djokovic in the last four in 2016 before losing to Rafael Nadal at the same stage past year.More news: US & China share heated exchange at Shangri-La Dialogue
"There is no point where I would say that it's not working". "It will be a pleasure to play against Novak".
Cecchinato has never won a tour-level match on a surface other than red clay; as it is, he entered this season with a career record of 4-23.
Thiem is into his third straight French Open quarterfinal, with two semifinals the last two years.
Keys next faces unseeded Yulia Putintseva of Kazakhstan, who beat 26th-seeded Czech Barbora Strycova 6-4, 6-3.
Murray last competitive match was in the 2017 Wimbledon 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. Mental, definitely not, because, you know, I'm in the quarterfinals of a Grand Slam.More news: Computex 2018: Intel Teases Audience With An Unnamed 28-Cores CPU
Men's fourth round: No. 1 Rafael Nadal beat Maximilian Marterer 6-3, 6-2, 7-6 (4); No. 3 Marin Cilic beat No. 18 Fabio Fognini 6-4, 6-1, 3-6, 6-7 (4), 6-3; No. 5 Juan Martin del Potro beat No. 9 John Isner 6-4, 6-4, 6-4; No. 11 Diego Schwartzman beat No. 6 Kevin Anderson 1-6, 2-6, 7-5, 7-6 (0), 6-2.
"First two sets were fantastic", Thiem said.
"At the end I was nervous".
"I remember I slid one time, and then I felt a muscle pull".
Having said this, Thiem's statistical edge is paper-thin and certainly not enough to justify a lower market price, so it is utterly evident that the market is factoring in a lack of faith in Zverev's ability to back up three consecutive 3-2 victories and in short, thinking that the German will be absolutely knackered from a fitness perspective.More news: Novak Djokovic OUT of the French Open after shock Marco Cecchinato loss