By Glenn Moore

Jannik Sinner is yet to find his best form this Wimbledon, but has nevertheless reached the last four. Should his next opponent be discouraged or encouraged by this?

The answer will become clear when the defending champion plays his semi-final on Thursday (local time), against either Novak Djokovic or Felix ​Auger-Aliassime.

Then he will surely need to up his game having struggled to put away Jan-Lennard Struff, the world No.74 on Tuesday.

At 36, and in his 47th grand slam, Struff was the oldest man in the professional era to reach his first major quarter-final.

Sinner beat the German 7-5 ‌7-6 (7-4) 6-3 but made 26 unforced errors and had to save a set point in the second set.

“A very, very tough player to play against,” said Sinner. “He deserves everything he has done and achieved in his career. Great person off the court.

“In the beginning I felt like he started better than me, I was struggling a bit. I tried to get into the match, I was serving a bit better, tried to stay there mentally, and of course very happy to be back in the semi-finals here.”

On another hot day in London there was no sign of a return of Sinner’s difficulty with the heat that caused his shock second round exit in Paris last month.

“We worked a lot, especially after Paris, trying to understand what went wrong there. It was a huge test today. I felt really comfortable on the physical side today so a big step forward.”

In the quarter-final suspended on Monday night by Wimbledon’s 11pm curfew second seed Alexander Zverev dropped the set in progress but still beat 13th seed Czech Jiri Lehecka.

Leading 6-4 7-5 3-3 overnight Zverev failed to win another game in the third set. The fourth went with serve until the tie-break, which the recent French Open winner took on his third match point, having double-faulted on his second.

Zverev, who will now play sixth-seed Taylor Fritz on Wednesday, said: “He (Lehecka) came out swinging but I’m happy to be through in four sets and not having to play another today.”

Meanwhile, Coco Gauff was 15 when she announced herself as a future Wimbledon champion, coming through qualifying, beating five-time winner Venus Williams and reaching the fourth round.

That was seven years ago. Until this week, while she has won grand slams in Paris and New York, the fourth round remained her high-water mark in London.

On Tuesday (local time) she finally booked a semi-final place, defeating fourth-seed Jessica Pegula 4-6 6-3 6-3 to become the highest-ranked woman left in the draw.

She is also, with Naomi Osaka losing to ninth seed Karolina Muchova 7-6 (7-4) 6-4, the only player left to have won a grand slam.

The seventh-seeded Gauff began badly on Centre Court on another baking hot day in south west London. From 40-0 up in the opening game she lost the next five points, including a pair of double-faults.

After breaking back in the sixth game, Gauff was immediately broken to love with two more double-faults.

Another double-fault followed at the start of the second set but Gauff recovered to hold and then her serve began to fire. Pegula, on her Centre Court debut, was the one to crack and it went to a decider.

Gauff broke early, Pegula responded, but Gauff broke again to take the tie.

“Honestly, it’s pretty insane, considering I hadn’t won a match on grass in two years before this tournament,” Gauff said during her on-court interview.

“It feels really special considering the results I’ve had of late ‌and just especially ‌on this surface,” said ⁠the 22-year-old later.

“I’m able to ​relax a bit because, I feel regardless of how the rest of this tournament goes, I really think I’ve found a bit of a breakthrough on grass.

“In the past there’s commentary on my game, how maybe it doesn’t mesh with this surface, things like that,” she ​said.

“I ​think just trusting myself, trusting that my groundstrokes are ​good enough to be with anyone on this surface.

“Obviously I’m not satisfied. I want to go all the way.”

On No.1 Court, where Osaka last week beat Australia’s Daria Kasatkina, the four-time grand slam winner appeared to have serious intent when she reduced her walk-on outfit to just the train.

But after squandering a break chance at 5-5 she made too many errors in the tie-break and went a set down.

The 14th seed failed to exert much pressure on Muchova’s serve in the second set and was broken in the ninth thanks to two double faults and a wild volley.

“I played three times on this court and it was 0-3,” Muchova said. “I had a not good relationship with this court, but I am super happy we finally made it and I got the win.”

Wednesday’s women’s quarter-finals feature Marta Kotyuk (Ukraine, 12th seed) against Jasmine Paolini (Italy, 13th), and Linda Noskova (Czech, 9th) playing Elise Mertens (Belgium, 25th).

Whoever comes through will be a debutant winner and if it is Muchova or Noskova they will be the third Czech Wimbledon winner in four years after Marketa Vondrousova and Barbora Krejcikova.