NOVAK Djokovic is back at the top of the tennis heap. On September 9th the 31-year-old Serb defeated Juan Martín del Potro in straight sets to win his third US Open and 14th overall grand-slam title, moving him into a tie for third place on the men’s Open-era list with Pete Sampras. The only two ahead of him are his long-time rivals Roger Federer, who has 20, and Rafael Nadal, with 17. The championship in New York solidifies the Serb’s return to the elite, and brings back a question that had disappeared last year: can Mr Djokovic retire as the greatest of all time? It is easier than ever to imagine that he will.

12 months ago, the idea seemed doubtful. After losing his place at the top of the world rankings to Andy Murray at the end of 2016, Mr Djokovic sputtered through the first half of 2017, parted ways with long-time coach Marian Vajda in April and halted his season after Wimbledon to allow a lingering elbow injury to heal. His comeback in early 2018 raised more questions than it answered, as he failed to reach the quarter-finals of any of the first six tournaments that he entered. He reunited with Mr Vajda in April, and a few months later he finally added to his trophy cabinet by claiming his fourth Wimbledon title, after conquering Mr Nadal in a five-hour semi-final battle.

These back-to-back major titles, coupled with a victory at the Cincinnati Masters in August, when Mr Djokovic defeated Mr Federer in a straight-set final, mean that the Serb is currently the man to beat on hard and grass courts. (Mr Nadal remains the dominant force on clay, on which he has won 16 of 23 meetings against Mr Djokovic.) Because of his lacklustre start to 2018, Mr Djokovic sits at number three in the world rankings. But according to Elo, a rating system originally designed for chess that takes into account the quality of opponents and injury lay-offs, the Serb is now second only to Mr Nadal, and on hard courts, he is once again the best in the world.

The thought of Mr Djokovic returning to form should leave his fellow players quivering in their sneakers. His seasons in 2011 and 2015, in which he won three of the four grand slams and sustained winning percentages above 90%, are among the greatest recorded by anyone. In April 2016 he posted the highest Elo rating of the Open era, peaking at 2,460 points. A 100-point advantage in Elo ratings translates to a 64% chance of winning a match and a 200-point edge indicates a 76% probability. Mr Djokovic’s maximum figure is 220 points higher than Mr Nadal’s current table-topping mark of 2,240, meaning that the Serb at his zenith would be expected to beat the current version of the Spaniard roughly four times out of five.

Mr Federer’s highest-ever Elo rating, which he reached in 2007, was 2,406, and Mr Nadal’s, achieved in 2009, was 2,388. Only two other players, Bjorn Borg and John McEnroe, have ever exceeded 2,400. Mr Djokovic’s otherworldly level of play has often manifested itself in the Association of Tennis Professionals (ATP) Masters series, a group of nine second-tier events. He has won 31, a career tally second only to Mr Nadal’s 33, and is the only player to earn the “career golden masters” by winning each of the nine tournaments at least once.

That Mr Djokovic is still chasing the grand-slam totals of Mr Federer and Mr Nadal, despite competing at so high a level, is due to bad timing. Together with Andy Murray, the three players combine to make up the “Big Four,” the most dominant quartet ever to share the tennis stage. Mr Federer, the oldest of the group, began piling up Hall-of-Fame credentials against a more pedestrian field while the others were playing junior tournaments. Mr Nadal, a clay-court prodigy, claimed his first French Open championship in 2005, at age 19, two years before Mr Djokovic even reached a major semi-final. 13 of the Serb’s 14 grand slams have come since 2011, coinciding with Mr Murray’s peak and a period in which Mr Federer and Mr Nadal have remained at elite levels. When major titles are adjusted for the quality of competition—a technique introduced on this blog after last year’s US Open —most of Mr Djokovic’s 14 majors rate as considerably more difficult than average. His quality-adjusted haul is equivalent to 16 grand slams, whereas Mr Sampras’s 14 titles are revised down to only 11.7. An adjusted total of 16 still leaves Mr Djokovic in third place, but it narrows the distance between him and the top.

Mr Djokovic is well positioned to close that gap further still in 2019, starting with the Australian Open in January. Mr Federer, the defending champion, is now 37 years old, an age at which most players have long since moved into the commentary booth or the coach’s box. Mr Djokovic’s victory against the Swiss in last month’s final in Cincinnati was his third in a row against his old rival. Mr Nadal, who will likely be the top seed in Melbourne, retired from his US Open semi-final match against Mr del Potro with a knee injury that has recurred throughout his career. With Mr Murray struggling to return from his own ailments and no other strong contenders in the field, Mr Djokovic may soon get a taste of the middling level of competition that Mr Federer enjoyed before the rest of the Big Four emerged. An early Elo-based forecast of the 2019 Australian Open predicts that Mr Djokovic has a one-in-four chance of scoring his 15th major title in Melbourne, even if Mr Nadal is at full strength. Should the Spaniard’s knees prevent him from competing, Mr Djokovic’s odds would improve to one-in-three. If the Serb remains at his current level, each subsequent hard- and grass-court grand slam could become even easier, as Mr Federer’s age and Mr Nadal’s injuries decrease the likelihood that those two men can remain at the top of their own games.

Had Mr Djokovic’s slam-winning career come to a close after his struggles in 2017, he would forever have been known as the third-best of a great triumvirate. Due to the central place that the grand slam-leaderboard holds in the tennis world, he still faces a mighty challenge to be seen as more than that. Only nine other men have won at least seven majors in their entire careers, and Mr Djokovic needs to add that many simply to surpass Mr Federer. Even the Swiss, who has had a more productive fourth decade than any other male player in history, has only added three majors after his 31st birthday. But Mr Djokovic is playing better in his 32nd year than Mr Federer did, and his more rounded game means that he can compete on all surfaces—even with Mr Nadal on clay—in a way that Mr Federer could not. The odds are against the Serb reaching 20 majors, but another two or three seasons at the top could easily give him a final total of 17 or 18—enough to move Mr Djokovic out of his default position in third place. Fans of Roger and Rafa have long dominated the debate about who is the greatest male player of all time. But by the end of the decade, Mr Djokovic’s trophy cabinet could well be as bulging as those of his legendary rivals.