Former Wimbledon finalist Berrettini, who has struggled with various injuries in recent years, called it quits in the second set when trailing 7-5, 5-2 on Court Philippe Chatrier.
“You never want someone to end the tournament like this,” said the 25-year-old Arnaldi.
“I’m sorry for him and I hope he’s going recover and soon it’s going to come the grass and he’s going to be very tough to play.”
Arnaldi has spent the largest time on court for a player en route to a Grand Slam semi-final, so will likely be grateful for the extra rest ahead of facing 10th seed Cobolli on Friday.
The world number 104, who himself was sidelined with injury earlier this season, had played back-to-back five-setters in the third and fourth rounds, totalling over 10 hours.
“Unbelievable, I still can’t believe it if I think where I was one month ago, I was nearly 150 in the world,” he added.
“I’m tired, that’s for sure, but you know I train and I play tennis to play these kind of tournaments, these kind of matches.
“Obviously today I was a bit more tired than usual… but I’m here.”
Berrettini, ranked 105th, had enjoyed a brilliant tournament in his first appearance at Roland Garros since 2021.
But he had also played some demanding matches, including the longest of his career in the last 32 against Francisco Comesana, and his efforts finally took their toll in the last night session of the event.
“I have to take unfortunately the right decision,” said Berrettini.
“Even though it was obviously a big stage, I’m the last one that wants to retire. I’m so tired of it. I just I don’t want to do it, but sometimes you have to do it.”
Berrettini got off to a dream start by racing 3-0 up with two breaks of serve, but Arnaldi eventually ground him down in a marathon, 82-minute opening set.
Arnaldi continued his momentum into the second, quickly moving 2-0 ahead only to gift his break straight back.
However, there was a painful moment for the injury-plagued Berrettini, as he started grimacing and called for a medical time-out.
Berrettini was broken again as Arnaldi moved 4-2 in front, and after failing to chase down another winner from his opponent and then stopping to clutch his left hip, his coaching staff started waving at him to stop.
He reluctantly shook hands during the next changeover, putting Arnaldi into the last four of a men’s draw thrown wide open last week by the second-round exit of their compatriot Jannik Sinner. (AFP)
