Andy Murray and Daniil Medvedev sealed their place in the Qatar Open semi-finals while top seed Andrey Rublev was upset 4-6 6-4 6-3 by Jiri Lehecka.
Rublev won this tournament in 2020 and looked to be on his way to the final four after a strong opening set put him ahead.
But Lehecka battled back in the second to level the match and then found a crucial break in the second game of the deciding set to seize the advantage.
It was a lead he would not relinquish, holding his nerve despite watching a pair of match points come and go to clinch victory and knock out the world number five.
Lehecka will meet Murray in the semi-finals after the Brit came from a set down to see off Alexandre Muller 4-6 6-1 6-2 and keep his hopes of winning the Qatar Open for a third time alive after back-to-back titles in 2008 and 2009.
A tough opener had Murray facing a deficit against the world number 170, but a typically gutsy display saw him pull off the comeback and reach his first tour-level semi-final since June 2022, when he lost in the Stuttgart Open final to Matteo Berrettini.
Medvedev sealed his place in the final four with a hard-fought 6-2 4-6 7-5 win over Christopher O'Connell.
The world number eight triumphed for the seventh straight match, but did not have it all his own way against his Australian opponent, needing a crucial break with the final set tied at five games each to put himself back into the ascendancy.
Felix Auger-Aliassime awaits Medvedev in the next round after prevailing over Alejandro Davidovich Fokina in straight sets.
The world number nine out-aced his opponent eight to two to author a 6-4 7-6 (7-5) victory after one hour and 48 minutes, and he will look to improve on his dismal record against Medvedev on Friday, having lost all five of his meetings against the Russian.
At the Open 13 Provence, top seed Hubert Hurkacz reached the quarter-final after rallying from a set down to see off Leandro Riedi 4-6 6-3 6-2.
Second seed Jannik Sinner was forced to withdraw through illness, sending wildcard Frenchman Arthur Fils into the final eight, where he will meet Stan Wawrinka.
Alex De Minaur teed up a clash with Benjamin Bonzi by cruising past Alexander Ritschard 6-3 6-3, while world number 25 Grigor Dimitrov will play Alexander Bublik on Friday after beating Gijs Brouwer in straight sets.