The men’s semifinals of the U.S. Open are annually appointment television for tennis fans around the world. But in the case of Friday’s second semifinal, it’s the tennis match that no tennis fan on the planet wanted to see — except, perhaps, for those in Argentina. To be sure, all signs point to Rafael Nadal and Juan Martin del Potro having a highly competitive, entertaining semifinal — del Potro is actually a respectable 5-8 all-time against the No. 1-seeded Nadal. But this one was supposed to be Nadal versus his longtime rival Roger Federer, on the hard courts of Flushing for the first time ever.Federer-Nadal at the U.S. Open is tennis’s white whale — and once again, it slipped away.On Monday, Federer and Nadal appeared to be on a collision course; both reached the quarterfinals, and both were in the same side of the bracket. Nadal held up his end of the bargain, beating Andrey Rublev in straight sets to secure a semifinal berth. But late Wednesday, Federer was unceremoniously defeated by the Argentinean del Potro, ending his bid for a third Grand Slam championship of 2017 — and crushing the hopes of everyone who has followed the game in the past 15 years.Along with Novak Djokovic, Federer and Nadal are the two most successful players of their era.1If not any era. Between them, the pair have won 34 Grand Slam titles and appeared in the semifinals of 33 other major tournaments. They’ve gone head-to-head 12 times at the other three Slams — three times at Wimbledon, four at the Australian Open and five times at Roland Garros. So it’s astonishing that their paths have never crossed at the U.S. Open. Do the math, and the odds of the colossal clash never happening are minuscule.Let’s focus on Grand Slam semifinals, since all of the actual Rafa-Fed matchups have happened at that stage of a major or later. Federer first made the semifinals of a Grand Slam in 2003, and since then, he’s advanced at least that far in 74 percent of all non-U.S. Open majors. Nadal made his semifinal debut in 2005 and has made it that far in 53 percent of non-U.S. Open majors ever since. So, if we simply used the other three majors as a guide, we’d expect Federer and Nadal to have about a 39 percent chance of simultaneously making the semifinals of any given U.S. Open. And in non-U.S. Opens when both made the semis, they ended up facing each other 86 percent of the time, whether in that round or the final.Based on those numbers, then, we’d expect there to be a 34 percent chance that Federer and Nadal would end up facing each other at a given U.S. Open. (Indeed, they’ve faced each other 12 times in the 38 non-U.S. Open Slams since 2005, including their first head-to-head meeting at Roland Garros — good for a rate of 32 percent.) If we apply that 34 percent mark to each of the 13 U.S. Opens staged since 2005, we would expect there to be only a 1-in-204 chance (0.5 percent) that they wouldn’t end up facing each other at least once in that span. So the odds that we would never get this U.S. Open matchup are truly microscopic.Now, some of this can be attributed to both Federer and (especially) Nadal enjoying comparatively less semifinal success at the U.S. Open than at the other three majors. Since his first semifinal breakthrough at a Slam, Federer has only made the semis 67 percent of the time on the Flushing hardcourts, and Nadal has only made it that far 38 percent of the time. If you use those rates instead of the non-U.S. Open numbers as your baseline, the odds against ever seeing a Federer-Nadal clash rise to 4 percent.Still, even that is a fairly small probability. Any way you cut it, it’s positively shocking that American audiences have never been treated to seeing two of the greatest players ever play each other in person before.