In our sports coverage at FiveThirtyEight, we’ve referred several times to regression to the mean. It’s an unromantic truth about outstanding sports performances: They’re usually the product of skill and determination, but also a healthy dose of luck. We might expect the outstanding play to continue because of skill and determination, but the luck is just as likely to turn on its head as to persist.One sports figure, in one context, has upended the idea of regression to the mean more than just about anyone else over the past nine years: Rafael Nadal, when he plays tennis on clay. His losses have been so few and far between — and usually against one of the greats of all time, or while he’s hurt, or both — that they have seemed like the exceptions that prove the rule. From April 11, 2005, to April 17, 2014, Nadal went 260-9 on clay, winning 96.7 percent of his matches. That’s better than the best roughly nine-year runs on a surface of Nadal’s closest historical peers — Roger Federer or Pete Sampras on grass, Bjorn Borg on clay. Nadal may not be the best player in the group, but he’s probably the best player on any single surface in history.Then, suddenly, regression struck back — in a big way. Nadal, 27, has lost in consecutive weeks in clay-court quarterfinals, in tournaments he’s dominated, to good but not all-time-great opponents. Nadal still could run the table at the big remaining clay events in Madrid and Rome, and the French Open in Paris, but his footing on clay hasn’t looked so precarious since he was 18 years old.In the nine years until last week, Nadal had lost nine times on clay. Five times he lost to Roger Federer or Novak Djokovic, his top rivals and both all-time greats. Another loss came when Nadal’s foot was injured against Juan Carlos Ferrero, a former world No. 1 and 2003 French Open champion. Nadal’s only loss at the French Open, to Robin Soderling in 2009, was his last match before missing two months with a knee injury. He lost to Horacio Zeballos last year, in Nadal’s first tournament back after another seven months off-tour with knee problems. And he lost to Fernando Verdasco in 2012 in Madrid, the only tournament ever played on blue clay. After that loss, Nadal said: “This surface destabilizes the game. It is a completely different game, and I don’t want to take risks.”But this month, Nadal has no evident injuries, and he was playing on red clay. He was also facing opponents whom he’d previously dominated on the surface. Last Friday in Monte Carlo, where Nadal had been 50-2 in his career, he lost to David Ferrer after beating him 17 straight times on clay (a streak dating to before Nadal’s 19th birthday). And Friday, in Barcelona, where Nadal had been 42-1 in his career, he lost to Nicolas Almagro. Nadal had beaten Almagro in all 10 of their meetings, on all surfaces, while losing just two sets. Each loss came in a quarterfinal, a round in which Nadal had won 45 straight matches on clay.Twice in his nine-year run of dominance, Nadal lost on clay in consecutive tournaments: in 2009, to Soderling and Federer; and in 2011, twice to Djokovic. Immediately after each pair of losses, Nadal went on big clay-winning streaks: 37 straight matches after his 2009 rut, and 22 straight starting in 2011. He may need another such run to maintain his No. 1 ranking: His lead over Djokovic is fewer than 2,000 points, and Nadal has 4,000 points to defend in his next three tournaments. Djokovic has just 910 points to defend during that span. Last Friday, it looked as if Djokovic’s hot form was the biggest threat to Nadal’s top spot in the rankings. Now, with Djokovic nursing a wrist injury, the biggest wild card in the race for No. 1 is Nadal’s sudden regression toward the mean.