The fact that Struff was able to test the Spaniard should also come as no surprise. Alcaraz led the head-to-head 3-1 coming into this match, though none of those victories over the German veteran were easy.
Both their Madrid Open final in 2023 and fourth round at the same event in 2024 went to three sets, while they had previously contested a five-set match at Wimbledon.
Alcaraz was pushed to the limit by Struff in their round-once clash back in 2022, and this proved to be another test for the two-time champion.
It was smooth sailing for the Spaniard early on, with the 22-year-old easing his way to a comfortable set and a break lead.
However, a quick spell of play quickly switched the momentum in Struff's favour, the world No. 125 racing away with the second set.
While the 35-year-old's level rose significantly for a short spell, two quick breaks of the Alcaraz serve highlight somewhat of an ongoing issue for the Spaniard.
Alcaraz's percentages on his service stats were far from his desired level in his round one and round two matches against Fabio Fognini and Ollie Tarvet, respectively, and his serve proved a key issue once again here.
After improving to a 65% first serve percentage against Tarvet in the second round, Alcaraz was down to 58% on Friday, a not-insignificant drop-off.
While he won 73% of first serve points and an improved 62% of points on his second serve, this is still an issue he is battling with during his Wimbledon title defence.
"Well, I knew at the beginning that it was going to be really difficult. I had to be really focused on every shot, on my serve games, on the return" said Alcaraz on-court.
"I think his game suits pretty well to the grass, big serve, approaching to the net as much as he can. I'm just really pleased about everything that I have done today."
Alcaraz will face his first-seeded player of the tournament on Sunday, with 14th seed Andrey Rublev waiting in the fourth round.
Rublev has had his struggles in recent months, with the Russian dropping out of the top 10 across the opening stages of 2025.
But he has found some consistency in recent weeks, with back-to-back second-week showings at both Roland Garros and Wimbledon; in Paris, he was beaten in straight sets by Jannik Sinner.
Alcaraz is the on-paper favourite, though he will surely hope some improved serving makes his life a little easier.