THE ALL ENGLAND CLUB, London — Just in time for the most intense part of Wimbledon, London is heating up.Jannik Sinner, the defending men’s champion and world No. 1 who has shown a propensity to melt in hot weather, regardless of the opponent, will take on Jan-Lennard Struff in the quarterfinals Tuesday, on an 85-degree afternoon in the British capital.If he can get through the 36-year-old German, who is a dangerous foe on the grass, he’s likely to play a much more familiar one in the semifinals: 24-time Grand Slam champion Novak Djokovic could be waiting for Sinner, 24, under a bright, 87-degree sun on Friday afternoon.Ahead of the tournament, Italy’s Sinner said he and his team have committed to more hot-weather training, to acclimate his body to a schedule that feels it grows hotter every year.But at this edition of Wimbledon, Sinner’s issues have so far been more prosaic, and the fix easier to find. His baseline game is out of rhythm. His serve is the vicious metronome keeping him on the straight and narrow.Why do players wear white at Wimbledon?Ava Wallace and Madison EadesSinner called out his serve after his fourth-round win over Shintaro Mochizuki, the world No. 151 from Japan. Sinner won the first set comfortably in 33 minutes, but the match appeared to tighten in the second set, when it headed into a tiebreak. If Mochizuki was going to have a chance to get his teeth into the match, this was it.Really, Mochizuki never had a chance, even if on the surface it looked like Sinner might be in for another long day, after having gone five sets against Serbia’s Miomir Kecmanović in the first round.In the second set against Mochizuki, Sinner landed over 70 percent of his first serves, and he won 24 of 28 points when he did. He smacked 10 aces. Overall, he played 38 points on his serve, and on 17 of them, he didn’t have to hit a second shot.He served on just three of the seven points in the tiebreak, which he won 7-0. An Ace. A kicker onto the T that had Mochizuki on the back foot, then a slice out wide that Mochizuki dumped into the net.“I felt like I was serving well at times, especially important moments today,” Sinner said in his news conference. “That helped me also being a bit more freer in the return games.”Therein lies the Sinner blueprint for the rest of the Wimbledon fortnight, if he is going to lift the trophy for a second consecutive year. He is not the player he was in the early clay-court season, when he won three consecutive tournaments — the Monte Carlo Masters, the Madrid Open and the Italian Open. He won the BNP Paribas Open at Indian Wells and the Miami Open before them, but even in that dominant hard-court run, there were signs of slippage.