Sinner survives Struff barrage to reach Halle semi-finals
Germany's Struff unleashed 18 aces and saved 16 of 18 break points but still went down to a 6-2, 6-7 (1/7), 7-6 (7/3) loss to the Italian.
"It was a very tough match," said Sinner. "I had chances in the second, had chances in the third, but couldn't use them.
"I just try to accept these kind of challenges."
Sinner will face either China's Zhang Zhizhen or Christopher Eubanks of the United States for a place in the final of the tournament, a key warm-up for Wimbledon which starts on July 1.
Germany's Alexander Zverev edged closer to his first grass-court title with a come-from-behind victory over Arthur Fils in his quarter-final.
French Open runner-up Zverev, the world number four, came through 6-7 (5/7), 6-3, 6-4 against his French opponent.
The 27-year-old, a two-time finalist at Halle, becomes the first man since Roger Federer in 2019 to reach back-to-back semi-finals at the tournament.
"After the French Open, I was hoping on grass I would play a bit quicker and not play for two-and-a-half, three hours every single match, but I’m continuing what I always do," said Zverev.
Zverev will take on 2022 champion Hubert Hurkacz for a place in the final.
Poland's world number nine defeated Marcos Giron of the United States 7-6 (7/5), 6-4 on the back of 42 winners, including 11 aces.
