The world number one was pushed at times by 36-year-old German Struff, but clinched an ultimately comfortable 7-5, 7-6(4), 6-3 quarter-final victory.
On Friday, Sinner will aim to reach his seventh Grand Slam final and first this year when he takes on either Djokovic or Canadian third seed Felix Auger-Aliassime.
He dealt well with 30C heat at the All England Club, after previous struggles in hot conditions, including when he was dumped out of the recent French Open in the second round by Juan Manuel Cerundolo after leading by two sets and 5-1.
"Thanks for reminding me," the four-time Grand Slam champion joked when asked about the heat.
"We worked a lot, especially after Paris, trying to understand what went wrong there.
"In any case, it was a huge test today, but I felt really comfortable in a physical sense.
"If it happens again, and I hope not, we know we need to change a couple of things again."
Sinner was beaten by Djokovic in the Australian Open semi-finals this year, having won their previous five meetings.
They have met three times before at Wimbledon, with Djokovic coming out on top in the 2022 quarter-finals and 2023 semis, before Sinner gained revenge in the last four 12 months ago.
Sinner has won his last five matches against Auger-Aliassime, dropping only one set in the process.
Struff started strongly, perhaps buoyed by pushing Sinner in a tight defeat on grass at Halle last month, but fell a set down after being broken in the 11th game.
Struff carved out a set-point opportunity in the second set but Sinner held him off as it went to a tie-break.
The top seed quickly moved 5-2 in front in the breaker and clenched his fist in delight when Struff sent a backhand long to give him a two-set lead.
Struff became the oldest first-time men's Grand Slam quarter-finalist in the Open era by reaching the last eight.
But his resistance was ended as Sinner won the last three games to wrap up the match.
Wimbledon 2026
Wimbledon is taking place from June 29th to July 12th at the All England Club in south-west London. With 128 players in the draw, it is the third of the four Grand Slams of the tennis season and the only one played on grass.
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