Grigor Dimitrov was forced to rescue two match points before eventually overcoming Malek Jaziri, while Rafael Nadal had no such problems at the Barcelona Open on Thursday.
Second seed Dimitrov, aiming for just his second clay-court title and his first in four years, was taken the distance - and to the brink of a shock defeat - before completing a 7-5 3-6 7-6 (10-8) success.
The opening set proved a sign of things to come as world number 88 Jaziri took a 4-2 lead, but Dimitrov hit back and belatedly forged ahead by converting his fifth set point.
Jaziri was unbowed, however, and broke at either end of the second to level the match before once again taking a 4-2 lead in the decider, and the Tunisian earned a match point at 6-5 with Dimitrov on serve.
He could only net a backhand, though, as the Bulgarian forced a tie-break, where the drama continued.
Dimitrov wasted a trio of match points - sending a makeable backhand wide on the third - and breathed a sigh of relief when Jaziri dumped a forehand into the tramlines at 8-7. As the match approached the end of its third hour the Tunisian finally cracked, netting a forehand as his opponent squeezed into the quarter-finals.
He will play Pablo Carreno Busta, who defeated Adrian Mannarino in another match decided on a final-set tie-break.
There was no such drama later in the day as Nadal breezed past compatriot Guillermo Garcia-Lopez 6-1 6-3 to chalk up his 16th straight victory on clay, as well as extending his record of consecutive sets won on the dirt to 40. Martin Klizan awaits in the last eight.
Third seed Dominic Thiem got the better of Jozef Kovalik, while there were also wins for Stefanos Tsitsipas, fourth seed David Goffin and Roberto Bautista Agut.
Meanwhile, there was a shock at the Hungarian Open as top seed and defending champion Lucas Pouille was dumped out 6-3 6-4 by John Millman. Yannick Maden, Andreas Seppi and Nikoloz Basilashvili were Thursday's other winners in Budapest.