The Spanish champion knew all about the danger of the 35-year-old Struff, having been knocked out of the 2021 French Open by him when he was an emerging star and then also getting stretched by the towering German over five sets at the following year's Wimbledon.
And the same old problems emerged again on Friday when, after Alcaraz had eased through the first set, Struff, who blasted down 13 aces in all, responded brilliantly to clinch the crucial break for 5-3 before levelling the match.
But Alcaraz, who'd also had a serious workout from another veteran, Fabio Fognini, over five sets in the opening round, once again found another gear to prevail 6-1 3-6 6-3 6-4 in two hours and 25 minutes.
"I knew it was going to be really difficult and I had to be focused on every shot," said the 22-year-old.
"His game suits the grass, big serves, coming to the net, so I'm pleased with everything I did today. Proud to get the win in four sets.
"To be honest I was suffering in every service game I did. Lots of break points down. It was stressful," added Alcaraz, who set up a last-16 date with 14th seed Andrey Rublev, who eased past veteran French leftie Adrian Mannarino 7-5 6-2 6-3.
Taylor Fritz, who had already negotiated two marathon five-setters over three days to reach the third round, needed another three hours and 12 minutes to get past Spaniard Alejandro Davidovich Fokina.
Yet though his workload has been massive and he's had issues with his knee, he reckons he's actually feeling stronger as each match goes by as he gets ready to face Australian Jordan Thompson, four-set victor over Luciano Darderi, in the last-16.
It was the end of the road for Brazil's rising teenage star Joao Fonseca, who couldn't keep his legion of noisy fans happy after losing to the resurgent Chilean qualifier Nicolas Jarry, who secured a fourth-round meeting with Britain's Cameron Norrie after a 6-3 6-4 3-6 7-6 (7-4) victory on a raucous No.2 Court.
The home fans, fed up after the exit of their big men's hope Jack Draper on Thursday, still have Norrie, their 2022 semi-finalist, to cheer after the last British man standing beat Italy's Mattia Bellucci 7-6 (7-5) 6-4 6-3.
A surprising face in the last-16 draw will be Poland's world 109 Kamil Majchrzak, who beat Frenchman Arthur Rinderknech 6-3 7-6 (7-4) 7-6 (8-6) and will next face Russian 17th seed Karen Khachanov, who finally beat Portugal's Nuno Borges in a match tiebreak 7-6 (8-6) 4-6 4-6 6-3 7-6 (10-8).
