It was at 5-all, 30-15 in the first set, Roger Federer serving, when Juan Martín Del Potro blasted a forehand that must have traveled 600 miles per hour. DelPo hit it so hard that, despite just okay placement, an already-sprinting-in-that-direction Federer couldn't even get a racquet on it. At his best, Federer just lets that point go, serves an ace on his next serve, and it's forgotten. But if you've watched Federer enough, you've seen points that seem to rattle the guy. (Think back to the missed overhead in the fourth set of the Australian Open final this year, after which Roger pretty much fell apart--speaking relative to his abilities, of course--until early in the fifth set, at which point he was already down a break.) So it was here. He double-faulted on the next point. At break point, he hit a so-so serve that he followed in to net, then hit a so-so volley to Del Potro's forehand side, which gave Del Potro an easy cross-court pass and the break. Del Potro held at 6-5 to take the set.
Federer seemed consistently off throughout the match. Still, as with his first- and second-round matches here at the Open, he seemed like he was going to escape. He won the second set and held four set points, including one on his own serve, in the third. Del Potro saved all four and won the third set 7-6 (8). After that, it was no great surprise to see Federer broken early in the fourth and lose the match.
Clearly my predictions aren't worth much. Though I predicted that Andrey Rublev didn't have the tools to challenge Rafa on this stage (Rafa proved me right, winning 6-1, 6-2, 6-2), I also predicted that DelPo wouldn't be able to recover from his illness and previous-round five-setter against Dominic Thiem. Nope. And before that, I predicted that Federer, having struggled so mightily against Tiafoe and Youzhny in the first two rounds, and clearly not 100%, would lose to Feliciano Lopez in the third round. But then Federer hit his stride against Lopez. He looked strong against Kohlschreiber. But it was here against Del Potro that Federer played the match I suspected he would against Lopez and got shown the door.
Nevertheless, I'll make another prediction. Nadal surely watched Del Potro's weak backhands, almost always crosscourt, and knows that he can send heavy crosscourt forehand after heavy crosscourt forehand to DelPo's backhand side, and that DelPo will be unlikely to have a response. In the other semi, both Anderson and Carreno Busta are both solid players, but neither is Rafael Nadal. At this point, the US Open is Nadal's to lose.