Anthony Gruppuso-USA TODAY Sports / Action Images

The first set of the US Open semifinal between Juan Martin del Potro and Rafa Nadal looked like a continuation of Del Potro’s quarterfinal win against Roger Federer two nights earlier.

Nadal was trying to stay away from Del Potro’s forehand and pick on his backhand, as Federer had, and was having similarly little success. Del Potro continued to hold steady off that wing, using it not just as a reliable defensive shot but occasionally getting opportunistic with it, using it to move Nadal around and stepping into a couple down-the-line winners. There were extended cross-court rallies in which Nadal’s forehand broke down before Del Potro’s backhand did.

“I was playing too much against his backhand,” Nadal admitted after the match. “I felt that he was waiting for me there.”

The instinct to stay away from that world-destroying forehand was perfectly understandable, but it often wound up backfiring. When Nadal didn’t get enough pace behind his shots into Del Potro’s ad court, the Argentine would just step around the backhand and uncork massive inside-out and inside-in forehands.

Meanwhile, Del Potro continued to dominate with his serve, winning 73 percent of his service points in the first set and facing nary a break point, with Nadal setting up to return from the back wall.

Del Potro won the set, 6-4. He won just five games the rest of the way.

The most important tactical adjustment Nadal made was beginning to attack to the deuce court. The forehand down the line is his bread-and-butter shot, and, recognizing that Del Potro would continue to cheat to get around to his own forehand, Nadal started cracking the ball into the deuce corner to open up the court. Suddenly, Del Potro found himself running after everything. Suddenly, he couldn’t find any opportunities to go on the offensive. It wasn’t long before he was dragging his feet and running on fumes.

“He played very smart from the second till the end of the match,” Del Potro said. “Because I was just standing all the time in my left side, and once he plays down the line, he won the point.”

Nadal was the aggressor from the moment the second set began, and everything got easier and more comfortable for him from there. He ripped off nine straight games to bagel Del Potro in the second and seize control of the third. He attacked from either wing – “his balls came too fast from both sides,” Del Potro said – and couldn’t seem to miss. In the final three sets, Nadal hit 36 winners and 10 unforced errors.

He started making his way to the net and ending points with overheads and nasty backhand volleys; he won 21 of 27 approach points for the match. He served lights-out, and was particularly devastating going down the T; he lost just 10 service points in the final three sets combined. He started creeping further forward on returns, and pouncing all over second serves; by match’s end, Del Potro’s winning percentage on service points had dipped to 57, and he’d been broken six times.

Nadal dominated the court-position battle, and the pole-axed Del Potro forehands that had been so destructive against Federer, and so many before him, couldn’t penetrate the Spaniard’s armor.

Del Potro didn’t play his best, and seemed to run out of steam after nearly two weeks of emotionally and physically taxing tennis. But Nadal would’ve been nearly impossible to beat for player in any physical condition on this night. For three magnificent sets, he was untouchable.