Defending champion Stefanos Tsitsipas is out of the Monte Carlo Masters, losing to Taylor Fritz in the quarter-finals.

The 24-year-old Greek, ranked third in the world, battled back in the second set but was eliminated in just over an hour.

Fritz, 25, will face Russian Andrey Rublev in the semi-finals on Saturday.

Second seed Tsitsipas had beaten Fritz in three previous hard-court meetings, and had also recorded 12 victories in total on the Monaco clay, dropping only one set.

He levelled the match at 4-4 in the second set but lost out on his serve again to go behind 5-4, before Fritz sealed the win with an ace on his first match point.

"My level was very high, I played very well," said Fritz, who becomes the first American to reach the last four in Monte Carlo since Vince Spadea in 2003.

In the earlier quarter-final, Rublev secured a 6-1 7-6 (7-5) victory against German qualifier Jan-Lennard Struff.

The 25-year-old lost the first game on his serve before winning six in a row in a quick opening set against the world number 100.

But Struff, who defeated fourth seed Casper Ruud in the last 16, forced a tie-break in the second set, in which Rublev regained his composure after a nervous display to win on his second match point.

Miami Open champion Daniil Medvedev faces Danish sixth seed Holger Rune, and 16th seed Lorenzo Musetti takes on fellow Italian Jannik Sinner in the remaining quarter-finals on Friday.