Jan-Lennard Struff made life difficult for Carlos Alcaraz, but the reigning Madrid Champion eventually made it through. (More Tennis News)
Alcaraz had won both of his opening games with the minimum of fuss, but he needed three sets to get beyond Struff, who won a second-set tie-break.
However, Alcaraz eventually prevailed in the decider, clinching a 6-3 6-7 (5-7) 7-6 (7-4) victory.
Alcaraz, who is in the hunt for a third straight title in Madrid, will now face seventh-seed Andrey Rublev for a place in the last four.
Data Debrief: Prodigal son matches the master
There have been plenty of comparisons between Alcaraz and his fellow Spaniard Rafael Nadal.
And with his win on Tuesday, Alcaraz matched Nadal's record for the longest winning run at the Madrid Open, at 14 matches, since the tournament began in 2002.
