World number one Novak Djokovic suffered a 6-3 4-6 6-2 quarter-final exit against Russian Daniil Medvedev at the Monte Carlo Masters.
The 31-year-old, winner of the event in 2013 and 2015, lost to the world number 14 in two hours, 20 minutes.
Medvedev faces another Serb in the last four, world number 48 Dusan Lajovic.
Defending champion Rafael Nadal, seeking his 12th Monte Carlo title, came from 4-1 down to beat world number 35 Guido Pella 7-6 (7-1) 6-3.
Djokovic, who won a record seventh Australian Open title in January, lost in the third round at Indian Wells and the fourth round in Miami last month.
He had beaten Medvedev in the last 16 en route to his Melbourne triumph, but surrendered his serve five times as the 23-year-old 10th seed recorded his first victory against the 15-time Grand Slam champion in their fourth meeting.
Second seed Nadal won four successive games to serve for a fluctuating opening set with fellow left-hander Pella, but the Argentine broke again as Nadal conceded five games in a set for the first time in his last 24 sets at the Monte Carlo event.
Pella also served for the set at 6-5 but Nadal converted his third breakpoint opportunity to force a tie-break, which he completely dominated, taking the first six points.
Pella, 28, four times a beaten clay court finalist before he captured his first ATP title on the same surface at last month's Brazil Open, was then broken in the first game of the next.
He had a total of four breakpoint chances to level at 2-2 but Nadal averted them and played some typically blistering forehands as he went on to win, like Medvedev, in two hours, 20 minutes.
Nadal's semi-final opponent will be Italian Fabio Fognini, who played with a bandaged right arm and fought back to oust Borna Coric of Croatia 1-6 6-3 6-2.
The 31-year-old world number 18 has lost 11 of his 14 matches with Nadal but is one of only seven players to have beaten the Spaniard on clay on more than one occasion.
In the doubles, Britain's Jamie Murray, third seed with partner Bruno Soares, moved into the semi-finals with a 7-5 6-2 win over Diego Schwartzman and Joao Sousa.
Murray and Soares will play Dutch pair Robin Haase and Wesley Koolhof, who beat top seeds Pierre-Hugues Herbert and Nicolas Mahut in the opening round.