NEW YORK — Alexander Zverev has continually fallen short of his own expectations on the largest stages in tennis, but he’s playing and thinking as if he’s finally ready to crack the challenging Grand Slam code at the US Open.

Zverev, 23, moved to the semifinal stage on Tuesday, his monstrous serves echoing in the empty reaches of fan-free Arthur Ashe Stadium. The 6-foot-6 power server overcame a wobbly start to win his quarterfinal match against Borna Coric, who held a 3-1 lead in their series and had bolted out front in the match by a set 4-2.

This time, Zverev did not bolt for an exit, another Grand Slam chance wasted.

“I thought to myself at that point [in the second set] that I had nothing to lose,” Zverev said afterward. “It’s not a secret I didn’t play my best, but I found a way to win and that’s the most important. I feel like I can still improve a few more things and that only gives me confidence.”

Analysts watching that match sensed this was a watershed moment for Zverev.

ESPN’s Brad Gilbert said, “I saw something in him today that’s big for him. It was the kind of match that a lot of guys lose. It was ugly, but he won.”

A player with a sharp mind and the habit of saying what he really thinks, Zverev concurred. He acknowledged that Novak Djokovic‘s fourth-round departure via default opened the door for himself — and all his remaining rivals — to become the first newly minted Grand Slam champion in six long years of domination by the Big Three.

“The Novak news shocked us all,” Zverev said. “Obviously for us younger guys, we see that as a massive opportunity.”

Among those hopefuls: one of the least-heralded members of the deep Spanish contingent in the ATP, No. 27 Pablo Carreno Busta. The 29-year-old celebrated his reprieve from having to face Djokovic by playing the match of his life against Canada’s Denis Shapovalov on Tuesday night. That one lasted over four hours and ended at around 1 a.m.

Zverev occupies a niche in the game that is both unique and puzzling. He exploded onto the scene in 2017 and won two ATP Tour titles while still a teenager. He rode that boomer of a serve to an upset of Roger Federer before turning 20 (he still holds a 4-3 series edge on the Swiss icon). Zverev was unable to compete in the 2017 season-ending Next Generation ATP Finals showcasing the best 21-and-under talent in the ATP Tour because, already ranked No. 4 in the world, he qualified for a place in the elite eight-man field of the ATP World Tour Championships.

He was, all acknowledged, a rare talent. By the spring of 2019, Zverev was — and still is — the only active player outside the historic quartet of Federer, Djokovic, Rafael Nadal and Andy Murray with that many Masters titles.

But, oh those Slams. Those frustrating, disappointing majors. Until this year, Zverev went as far as the quarterfinals on only two occasions at a major, both at the French Open (2018 and 2019). He took losses to pros he manhandled with impunity in conventional tour events.

Theories about Zverev’s failure to make a big statement at a major abounded, even after he marshaled his resources at a trying time and reached this year’s Australian Open semifinals before the tour went on hiatus due to the coronavirus pandemic.

Some, like Gilbert, believe that the domination foisted on the game by the reigning icons was just too formidable an obstacle — for everyone. “If the [Big] Three weren’t here, we would not be in this situation. But nobody really has been capable of beating two of them in the same event, never mind three.”

Others think that Zverev wasn’t fit enough, or suggested that he wasn’t mature enough, emotionally or mentally, to complete a slog of seven potentially lengthy five-set matches against an unpredictable series of opponents. His attention span was too short in the eyes of those who saw him as too streaky and volatile to stay the course over two weeks. The real reason might be simpler.

“For me, it was mental. I wanted it too much,” Zverev said. “I was trying too hard in Grand Slams. I needed to learn how to deal with that pressure. I needed to learn how to deal with those expectations of myself.”

The confession might come as a surprise to those who see Zverev as overconfident. It would be understandable: Zverev has been in the top 10 almost from day one of his career. Success is all he has known. He appeared comfortable as an heir apparent to the trio of icons, content to bide his time. But Zverev was beginning to slide back, dropping from a career high of No. 3 in late 2017, down to his current No. 7 as other young challengers flourished.

The clear warning sign that Zverev was increasingly in trouble was a growing tendency to misfire with his signature weapon, the serve. In 2019, he led the tour with 131 double faults in 10 Grand Slam singles matches leading into the US Open.

“He’s had times when it goes off and he loses to himself,” Gilbert said, adding that when Zverev won the ATP World Tour Championships in 2018, he was serving 140 mph rockets and few double faults. “When he’s like that, he’s really difficult to beat.”

Cursed with the service yips, feeling the pressure of rapidly improving young rivals such as Stefanos Tsitsipas and Daniil Medvedev, Zverev realized he needed a new coach. Although his father (also Alexander) has been the guiding light of his career, the team has looked to outside eyes on a few occasions — most notably those of former Grand Slam champions Juan Carlos Ferrero and celebrated “supercoach” Ivan Lendl. But neither was able to form a strong bond with the young, headstrong star.

Enter David Ferrer, the recently retired Spanish pro. They began working together on a trial basis last July, and now seem a solid unit. “He’s so unbelievably well-organized, and the work has been incredible,” Zverev said of Ferrer. “I enjoyed the practices. I enjoyed the practices more than I did in the last few years. It was a no-brainer for me [to hire him].”

Ferrer did not travel to the US Open, but he has been advising Zverev remotely from Spain via videoconference. After formally joining the team, Ferrer told Spanish tennis website Puntodebreak: “He [Zverev] is very professional, both physically and on the court. The weeks we were together he worked at a high level, always very receptive to everything I said to him … they [the Zverev team] let me work on everything I wanted without any kind of obstacle.”

Famed for his stamina and unrelenting, baseline style, Ferrer quickly helped Zverev improve his strength and conditioning. But fixing the star’s serve was at the top of Ferrer’s to-do list. Zverev hit his low point in January at the ATP Cup, where his serving woes led him to turn to the stands and scream at his father during a live match broadcast.

Zverev managed his serve well in the subsequent Australian major, but the yips returned and his record was a tepid 6-5 at the time the tour went dark in March.

Power servers live and die by their first strike. Here at the US Open, Zverev hit on a novel remedy to his untrustworthy second serve at a time when the rest of his game was, at best, full of ragged edges. He began to hit the second with the same 130 to 140 mph gusto as his first delivery. The serve saved him against Coric, as Zverev converted an outstanding 71% of his first serves, while fending off 11 of a whopping 15 break points. Time and again, an atomic second serve rescued him.

If Zverev hopes to break through to win his first major, he’ll probably have to elevate his all-around game while continuing to serve the way he did for three sets against Coric.

“He has to come out with more energy, impose himself,” ESPN analyst Rennae Stubbs said. “He is such a big guy, has such a big forehand, beautiful backhand. He needs to bring that intensity and impose his personality, [say] ‘I’m coming at you. I’m gonna bring it today.’ He’s learning how to do that.”

Due to the absence of a major champion in the draw, Zverev will have to work hard to control and modulate his expectations. But all four men’s semifinalists on Friday will be in the same boat.

“Now I think is the time where it gets really interesting,” Zverev said. “We’re going to have a new Grand Slam champion. We don’t know who is, but lots of guys want it.”