Zverev was soon indicating his discomfort and he felt more pain when Thiem went 4-3 up with a majestic one-handed backhand victor after a double fault gifted him two break points.
"I thought about it [retiring]", Zverev admitted. Incredibly, this was nearly replicated by Fabio Fognini, who looked down and out after two sets, only to take his match with Marin Cilic to a decider, which eventually was taken by the Croat.
He might well need to up that schedule considering his quarter-final 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.
He was also taken to five sets by Dusan Lajovic in the second round and saved match point against Damir Dzumhur in the third, but getting through is all that matters to him.
Djokovic continued his solid progress too as he powered to a 6-2, 6-4, 6-2 win over Spain's Fernando Verdasco, and the Serb says he knows Cecchiato quite well.
Zverev was serving for the set at 5-4, he soon trailed 0-40.
Up next for Keys is Kazakh Yulia Putintseva, who upset 26th seed Czech Barbora Strycova 6-4 6-3.
Russia's Darya Kasatkina and Denmark's Caroline Wozniacki speak with officials during their. She will now face compatriot Sloane Stephens for a place in the final.
Asked whether he could have envisioned, even as recently as April, that he would get this far at a major tournament, Cecchinato answered with one word, "No", before breaking into as wide a smile as can be. "It will be a pleasure to play against Novak".
He was joined within the quarterfinals by Dominic Thiem, his subsequent opponent, and the rejuvenated Novak Djokovic. That left the Austrian facing a hard challenge in Nishikori, but Thiem made fairly light work of it to again win in four sets, 6-2 6-0 5-7 6-4.
It is the first time Keys and Stephens have reached the last eight at the French Open.
The clay courts of Roland Garros host the second Grand Slam of the season.
She clinched the victory on her first match point with a forehand victor to reach the last four at Roland Garros for the first time.
On the second match point, Nishikori hit a return long to end the 2-hour, 28-minute struggle.
In a tight first set of a match played in overcast, heavy conditions, Thiem converted the only break point off a backhand victor in the seventh game before securing the opener with an ace.
Striking the ball cleanly and powerfully in flawless conditions on Court Philippe Chatrier, the big-serving Keys bagged the opener in the blink of an eye against an overpowered opponent offering little resistance.
Zverev had survived successive five-setters in the previous two rounds, including saving a match point against Damir Dzumhur in the last 32, but his tired legs and blistered feet again worked overtime to carry him over the finishing line against Russian Khachanov at Court Suzanne Lenglen.
"I have practiced with him many times in Monte-Carlo when he was part of the Ricardo Piatti Academy team, and he trained a lot where I live". "This definitely paid off, the hours in the gym every day".
Such a stance is pretty reasonable in truth, and went along with the adjustments that I made to the bare numbers to find my final model price, so any stance on this match should focus on whether a bettor thinks that Zverev's accumulated fatigue has an impact in this match.