Alexander Zverev, the 20-year-old tennis wunderkind now ranked fourth in the world, has everything a future champion could hope for. He’s tall — 6 foot 6 — yet coordinated. He has a strong serve and hits deadly forehands and backhands. Zverev already has six ATP titles, including two Masters titles he won by beating Novak Djokovic and Roger Federer in finals. There’s just one problem: At Grand Slam tournaments, he’s a dud.There’s no obvious reason why: Zverev looks and trains like a Grand Slam contender. His serve and steady strokes should dominate in best-of-five-set matches, and by all accounts he’s in excellent physical shape. Yet at major tournaments, he struggles and, inevitably, stumbles. In his career as a pro, Zverev has never gone beyond the fourth round at a major, and he only got that far once. He never reached higher than the third round at the Australian Open and French Open, and he topped out at just the second round in the U.S. Open.When you compare Zverev to other current tennis stars, his Slam failures stand out. Active men’s pros who have won majors produced a range of results in their early years.1Through the year in which they turned 20. Rafael Nadal, the most astonishing youngster of the bunch, won 17 ATP tournaments through his age-20 season, including one French Open title. Nadal leads top pros with the highest overall winning percentage, followed by Djokovic, Andy Murray, and then Zverev. Through last year, Zverev even had a winning percentage of .407 against players ranked in the top 10, which was better than that of Federer and Djokovic at the same age, though still only good for fifth best on our list of standouts.2Behind Nadal, Juan Martin del Potro, Stan Wawrinka and Murray.But at the Grand Slams, Zverev fades away. He has the second-worst winning percentage among this group, just barely ahead of Juan Martin del Potro. Djokovic, now the winner of 12 major titles, had reached one Grand Slam final before the end of his age-20 season. Andy Murray reached the fourth round at three of the Slams. Even Marin Cilic, who had a losing record in his early years, had a higher winning percentage than Zverev in Slams.Zverev’s early losses don’t seem to have anything to do with unlucky draws, either. Some matches, in fact, are the opposite: He blew wins that, by rankings, should have been his. In 2015, he lost the second round of Wimbledon to a wild card, the small, quick American Denis Kudla. In 2016, Zverev was seeded No. 27 at the U.S. Open when he lost to an unseeded Brit, Daniel Evans. Last year was the worst of all. At the French Open, he lost in the first round to Fernando Verdasco, a 33-year-old solid clay court player, but unseeded. And then at the U.S. Open, Zverev, ranked No. 4 in the world, couldn’t outhit the smaller and unseeded Borna Coric, who won in four sets. (Coric lost his next match.)So far at the Australian Open, Zverev has won two rounds fairly easily and will face another young opponent, 21-year-old Hyeon Chung, who is not seeded, in the third round. If Zverev gets through that round, he could next face Novak Djokovic, who is trying to make a comeback from his right elbow injury. In other words, this just might be an ideal Slam for a strong Zverev run.No matter what happens, though, Zverev has one thing working in his favor: time. Tennis champions are lasting longer these days, but Federer and Nadal won’t be around forever. And Zverev looks like he can still improve. If all goes well, maybe his Grand Slam story will change from struggles to dominance.