Alexander Zverev was given a thorough examination of his Paris Masters credentials by Grigor Dimitrov, but the fourth seed progressed nonetheless to reach the last eight.
Zverev, who last in last year's final, saw match points slip from his grasp in the second set as Dimitrov levelled the match, however, he eventually prevailed on Thursday.
World number one Novak Djokovic benefited from a walkover, while Daniil Medvedev also moved through to the quarter-finals of the ATP Masters 1000 event.
Final 8!@AlexZverev secures his spot in the QF following a thrilling match against Dimitrov that he defeats in three sets 7-6(4) 6-7(3) 6-3.#RolexParisMasters pic.twitter.com/ocH6zPkoz9
— ATP Tour (@atptour) November 4, 2021
Zverev outlasts Dimitrov
Olympic Games gold medallist Zverev had to dig deep for his 7-6 (7-4) 6-7 (3-7) 6-3 win over former world number three Dimitrov.
It was Zverev's seventh win on the bounce, but he was well aware of just how deep he had to dig to see off a resilient Dimitrov.
"Incredible match," Zverev, who won a fifth tour title of the year last week at the Vienna Open, said afterwards. "Grigor is playing very high-level tennis and I had to leave everything out there.
"I think it was a very high-level match and it could have gone both ways, especially the third set. I am happy with how things went in the end."
"It's been an unbelievable year that will have the perfect end to the season."
— ATP Tour (@atptour) November 4, 2021
@CasperRuud98 becomes the first Norwegian ever to earn a spot at the season finale #NittoATPFinals pic.twitter.com/LuoR5uAU5k
Djokovic moves through without playing
There was no such ordeal for top seed Djokovic, however.
Djokovic – a 20-time grand slam champion – received a walkover after Gael Monfils was advised not to continue with the tournament due to an adductor issue sustained in his previous win over Adrian Mannarino.
Taylor Fritz awaits five-time Paris champion Djokovic after the American saw off 10th seed Norrie 6-3 7-6 (7-3), while Casper Ruud will tussle with Zverev.
Ruud's emphatic 6-2 6-1 demolition of Marcos Giron clinched a historic achievement for the 22-year-old – who became the first Norwegian to qualify for the ATP Finals at the end of a breakthrough year in which no player can better his five tour-level titles.
Hubert Hurkacz did his chances of joining Ruud in Turin no harm as the Polish seventh seed – who holds the last automatic qualifying berth – fought back to defeat Dominik Koepfer 4-6 7-5 6-2 and set up a meeting with James Duckworth, who beat Alexei Popyrin 7-6 (8-6) 6-4.
Second seed and US Open champion Medvedev was another who had to come from behind before ultimately seeing off Sebastian Korda 4-6 6-1 6-3.
He will go up against Hugo Gaston, who became the first French qualifier to reach the Paris Masters quarter-finals since 1990 with his win over Carlos Alcaraz.
