“I think John’s serve is going to be jumping off the court today,” ESPN’s Brad Gilbert said as he sat courtside in Miami on Friday. It was a fair guess: The sun was out, the temperature was close to 80, and whether he’s knocking it flat or kicking it high, Isner’s serve always has a bite to it.

What Gilbert and the rest of us didn’t foresee was just how much spring Isner would have in his own step. After holding serve in the opening game, he jumped all over Delpo’s, jarring the Argentine and breaking him with a forehand return winner, a backhand ground-stroke winner, and a forehand volley winner. That was just the beginning for the American. Freed up after that early break, he hit 19 winners to zero from Delpo in the first set, and won it 6-1 in 28 minutes.

Del Potro, as Isner noted afterward, was “low on fuel” after winning two straight tournaments, in Acapulco and Indian Wells, and surviving three close sets against Milos Raonic on Wednesday night. On Friday, even an indefatigable pro-Argentine crowd couldn’t rouse him until late in the second set. Several of his first serves in the early going barely broke the 105-m.p.h mark—that’s less than what Isner put on a few of his supersonic forehands today.

Match point from Isner's win over Del Potro in Miami:

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But Isner’s eventual 6-1, 7-6 (2) win was less about Del Potro’s fitness than it was about someone finally using, and executing, the right tactical approach against him. Del Potro’s backhand has been his significantly weaker stroke for two years now, yet few opponents have gone after that shot as relentlessly as Isner did today.

The first point of the tiebreaker summed up the afternoon: Isner hit a hard return at Del Potro’s backhand, then hit a hard forehand approach to Del Potro’s backhand, and finished the point off with a drop volley winner that was again aimed at Delpo’s backhand. It took Del Potro until the eighth game of the match to hit his first forehand winner, a stat that was as much about Isner’s tactics as it was Delpo’s form. As Darren Cahill said, “John’s not letting him hit any forehands.”

So someone, for the moment, has solved Del Potro; his 15-match, two-tournament win streak is over. Afterward, he said he’ll skip the early part of the clay season, and may only return for Roland Garros. “I want to stay healthy for the whole season,” he says, and who’s going to argue with him.

As for Isner, he came to this event with just two wins in 2018; now he has reached his first final in Miami, and his third Masters 1000 final. Last season, he said he felt like he was improving with coach David Macpherson, and he looks as fit at 32 as he ever has. For a day, at least, the serve-bot turned everything-bot, and while he may have been the last person anyone expected to play the role of dark horse in Miami, he’s playing it pretty spectacularly at the moment.

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Isner solved the Del Potro puzzle in 6-1, 7-6 semifinal win in Miami

Isner solved the Del Potro puzzle in 6-1, 7-6 semifinal win in Miami

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