It was all square, with long tough rallies taking them to 3-3, but Thiem's backhand probed and powered for two break points in the seventh game, and he broke with another cross-court victor. Their match was suspended at 3-3 in the second set.
Alexander Zverev prevailed in his third consecutive five-set match at the French Open as he defeated Karen Khachanov 4-6, 7-6 (7-4), 2-6, 6-3, 6-3 on Sunday to reach a first career quarter-final at the majors.
At 2-2 in the fourth, Zverev saved two break points and then broke for 4-2 after Khachanov led 40-15 on serve.
But he put the second, a simple forehand victor, into the net and Zverev survived before going on to register a 6-2 3-6 4-6 7-6 (7/3) 7-5 victory in just under four hours.
Keys faces Kazakhstan's world No 98 Yulia Putintseva, who produced the upset of the day to eliminate 26th seed Barbora Strycova of the Czech Republic.
Reigning US Open champion Sloane Stephens ensured her progress to the final four by easing past 14th seed Daria Kasatkina 6-3, 6-1.
At world number 72, Cecchinato becomes the lowest-ranked man to reach the last eight in Paris since Latvian Ernests Gulbis (ranked 80) in 2008.
The 2016 champion Novak Djokovic was beaten 6-3, 7-6 (4), 1-6, 7-6 (11) by Italian Marco Cecchinato in a late quarter-final match, after a battle that lasted almost 3 ½ hours, which produced several scintillating rallies.
While fatigue may come into play, Zverev has alleviated concerns over his fitness, saying he is mentally strong to rally on in the ongoing tournament. "It will be a pleasure to play against Novak".
Novak Djokovic booked his spot in the quarters collecting his 200th career clay match win beating a Fernando Verdasco 6-3, 6-4, 6-2.
It was the Serb's 200th Tour-level clay court match win.
Macron reçoit Netanyahu pour parler de l'Iran
Mais pour Netanyahou, "cet accord est terminé" depuis la minute où les Etats-Unis ont décidé de s'en retirer. Sur ces points au moins, Benjamin Netanyahu et ses interlocuteurs européens pourraient se retrouver.
Keys said she was looking forward to Monday's blockbuster between Maria Sharapova - they share the same agent, Max Eisenbud - and Serena Williams.
Keys, the US Open runner-up to Stephens a year ago, will face Kazakhstan's Yulia Putintseva for a place in the semi-finals, after the world number 98 saw off Barbora Strycova 6-4, 6-3.
The seventh-seeded Thiem is in sight of a first final at Roland Garros.
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.
Then he won the next four points to give himself a set point - only to miss a smash from near the back of the court.
So it came as no surprise when the 21-year-old German began to cramp up badly and Thiem ending his entertaining run with a comprehensive 6-4 6-2 6-1 victory. "Bodily, clearly it isn't simple to play again to again to again five-set matches, however I'll handle it in some way.
I love these conditions; it was the first time for me on Court Philippe Chatrier not playing against Rafa, so I could enjoy the court".
Although Zverev triumphed in their Madrid meeting last month, Thiem has displayed the better form at Roland-Garros and should be the fresher of the two, so we're backing the Austrian to win in four sets and reach his third consecutive French Open semi-final.
"I'm not that young anymore".
"I've been down 2-1 every single time but I'm unbelievably happy to be into the quarter-finals".