THE RACE is on. Rafael Nadal and Roger Federer, the two men who dominated men’s tennis from 2005-10, have enjoyed a startling resurgence in 2017, splitting the year’s four grand-slam titles. Mr Federer picked up his wins at the Australian Open (where he defeated Mr Nadal in a five-set final) and Wimbledon, while his long-time rival paired his tenth French Open crown with this year’s US Open title. Mr Nadal cruised to the championship in New York, dropping only one set in his last four matches and polishing it off on September 10th with a routine win over South Africa’s Kevin Anderson, 6-3 6-3 6-4.

The latest trophy represents Mr Nadal’s 16th major title, second only to Mr Federer’s 19. Even as pundits have predicted for years that one or the other is finished as an elite player—especially Mr Federer, who turned 36 last month—there is no end in sight for this two-man race. The only other active player in range—Novak Djokovic, with 12—hasn’t claimed a victory since last year’s French Open, and is sitting out the second half of this season with an elbow injury. Andy Murray, the fourth member of the “Big Four” of men’s tennis, holds only three majors, and thanks to a hip injury, he too is missing much of the 2017 season.

The Spaniard’s fans can muster plenty of evidence to support the case for their hero as the greatest player of all time, but the three-major deficit is a challenging disadvantage to surmount. One of the best arguments left to Mr Nadal’s supporters is that Mr Federer padded his statistics in the weak era of 2003-07, before Mr Djokovic and Mr Murray reached their primes, and when a young Mr Nadal was primarily a threat on clay courts. The Swiss maestro enjoyed years swatting away lesser lights, whereas Mr Nadal has spent much of his career doing battle with Mr Djokovic and Mr Murray. On one hand, the most recent fortnight weakens this claim: Mr Nadal’s cushy path to the 2017 US Open title made even Mr Federer’s soft 2003 competition look like all-time greats. But ironically, on a difficulty-adjusted basis, the easiest to achieve of Mr Nadal’s major titles still nudged his career resume in grand slams just ahead of Mr Federer’s.

Mr Nadal’s draw in the last of this year’s majors was one of the weakest faced by any grand-slam winner in the past 30 years. During the first five rounds, Mr Nadal faced only unseeded players; in the semi-finals and finals, he beat the 24th and 28th seeds. Not only were many of the top players in the men’s game absent entirely—Stan Wawrinka, the defending champion, also missed the Open—but Juan Martín del Potro opened up the draw even more for Mr Nadal by eliminating Mr Federer, the pre-tournament favourite, in the quarter-finals.

Mr Nadal’s accomplishment may thus appear distinctly uninspiring when compared with his previous title runs. In the big picture, however, it was only a small step toward evening out his grand-slam draw luck with Mr Federer’s. At majors, the Spaniard has faced the other three members of the Big Four 13% of the time, and won a remarkable 74% of those meetings. In contrast, Mr Federer has played his Big Four peers in just 9% of his grand-slam matches, and won only 42% of them.

In and of themselves, these ratios do not prove that Mr Nadal has faced a harder road to his victories in majors. To evaluate the overall difficulty of grand-slam titlists’ draws, I used Elo—a rating system that assesses a player’s strength based on his won-lost record and the quality of his opponents—to measure the skill of an average major champion. I then estimated the probability that such a player would have won all seven matches against the opponents that each tournament’s victor had to face. For each win, I credit the champion with the difference between one and the Elo forecast: If an average slam champion on the tournament’s surface had a 90% chance of winning the match, the player gets 0.1 points (1 – 0.9); if a typical major winner would have gone in with a 20% shot, he’s assigned 0.8. Summing all the matches for each winner and applying the algorithm to the last several decades of grand slams results in an average credit of 1.23 per titlist, so I then divided each sum by 1.23 to normalise the results.

During most title runs, the champion tends to face at least one or two harrowing challenges. In the case of Mr Nadal’s draw in New York, however, Elo heavily favoured a typical slam winner against each of his under-powered opponents. By our new algorithm, the 2017 US Open title was worth only 0.62 majors—the lowest since the 2006 Australian Open (0.60) which, coincidentally, was claimed by Mr Federer. Mr Nadal hasn’t usually had it so easy: at the 2013 French Open, where Mr Nadal drew Mr Wawrinka and a peak-form Mr Djokovic in the quarter-final and semi-final rounds, the algorithm credits him with 1.65 majors. Over more than 40 years, only three grand-slam title runs have been more challenging.

Both Mr Federer and Mr Nadal have faced some easy draws and some hard ones—that’s the nature of a sport in which every tournament begins with an empty bracket and a lottery. But the lotteries—and the career trajectories of the strongest members of tennis’s supporting cast—have tended to benefit Mr Federer. The average grand-slam title run requires beating a set of opponents that the typical champion would defeat 23% of the time. Only eight of Mr Federer’s 19 major titles have come against competition more difficult than that. But 13 of Mr Nadal’s 16 championships have required him to confront harder-than-average obstacles. Suddenly, the difference between 19 and 16 isn’t as clear-cut as it initially seemed.

In fact, when we adjust those two numbers for difficulty, the King of Clay ends up with the strongest claim to the throne for the entire kingdom. On average, Mr Nadal’s titles are worth 1.18 majors apiece, while Mr Federer’s work out to 0.98 each. I’ll save you the multiplication: Mr Nadal comes out on top by the narrowest of margins, 18.8 to 18.7. The adjustment gives Mr Djokovic more credit as well, upping his total from 12 to 15.3 and swapping his fourth-place position on the traditional list with Pete Sampras’s third. It’s a promotion Mr Djokovic deserves, as all 12 of his major titles have required him to fight through tougher-than-average draws. Running the numbers also forces us to recognise just how hard Mr Wawrinka has needed to work to break the stranglehold his most fearsome peers have held at the grand slams. His three majors all rank in the top ten most difficult.

Even more than weeks atop the world rankings, Masters-level titles or peak Elo ratings—by which Mr Djokovic is the best-ranked, thanks to a run of five slam titles in six attempts between 2015 and 2016—difficulty-adjusted majors are unlikely to figure in the typical weekend argument about the greatest tennis player of all time. Yet for a single metric, it carries a heavy load, going to the heart of the case for Mr Nadal and cutting through much of the anecdotal carping that leads fans to discount one title or grant another extra credit. Each of the all-time greats has had their share of good and bad luck—little of it as good as Mr Nadal’s fortunes this past fortnight. As the sport’s greatest rivals continue their quest in 2018, it is important to remember that the Spaniard’s easy draw was an aberration, and that his career record in grand slams is every bit as good as Mr Federer’s.