After John Isner clinched his berth in the Miami Open final, he came over the top one last time, throwing a celebratory punch at a speed to match his thunderous serve.

The towering American moved one step closer to the most prestigious title of his 14-year career by beating a weary Juan Martin del Potro 6-1, 7-6 (2) in Friday's semifinals.

"I'm playing the best tennis I've played in a very, very long time," said Isner, 32. "And I'm so happy to be doing it here."

John Isner returns to Juan Martin del Potro, of Argentina, during a semifinal match at the Miami Open tennis tournament, Friday, March 30, 2018, in Key Biscayne, Fla. Isner won 6-1, 7-6 (2). (AP Photo/Lynne Sladky)

Dominating with his serve and aggressive baseline strokes, the 6-foot-10 Isner ended a 15-match winning streak for del Potro, who won Indian Wells two weeks ago by beating Roger Federer in the final. The toll of so many matches showed, and Isner was the better player from start to finish.

Seeded No. 14, Isner is 0-3 in ATP Masters 1000 finals. His opponent Sunday will be No. 4 Alexander Zverev of Germany, who beat No. 16 Pablo Carreno Busta of Spain 7-6 (4), 6-2.

Zverev, 20, has won two Masters 1000 titles, both in 2017.

John Isner celebrates after defeating Juan Martin del Potro, of Argentina, 6-1, 7-6 (2), in a semifinal match at the Miami Open tennis tournament, Friday, March 30, 2018, in Key Biscayne, Fla. (AP Photo/Lynne Sladky)

Isner raced to a 3-0 lead against del Potro, finished with 13 aces, never faced a deuce point on his serve and was at his best when closing out the victory.

He smacked consecutive aces to reach 6-all, and made all four first serves in the tiebreaker, including aces of 138 and 139 mph. He also hit two drop volleys for winners in the tiebreaker, with the second coming on match point.

If Isner looked relaxed, it's because he was.

John Isner, right, and Juan Martin del Potro, of Argentina, left, embrace at the net after their semifinal match at the Miami Open tennis tournament, Friday, March 30, 2018, in Key Biscayne, Fla. Isner won 6-1, 7-6 (2). (AP Photo/Lynne Sladky)

"I've played a lot of big matches where I wasn't able to free up," he said. "It's just a mentality. You've got to tell yourself to go for it. There's no other way to play the point, especially for me, because I can't really run from side to side."

Del Potro, seeded No. 5, was the fan favorite thanks to South Florida's large Latin population, and a flag-waving, ole-singing, near-capacity crowd tried to inspire a rally from the Argentine after he fell behind.

"I've been feeling tired in all my body for the last couple of weeks," del Potro said. "But I got so much love from the fans that the pains go away during the match."

Alexander Zverev, of Germany, serves to Pablo Carreno Busta, of Spain, during a semifinal tennis match at the Miami Open, Friday, March 30, 2018, in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)

Even so, Isner knew he was the fresher player. He held his own in the occasional extended rally, and repeatedly quieted the crowd with big serves.

"I was fortunate to get a del Potro who is very, very low on fuel," Isner said.

Isner beat three seeded players en route to the final, including No. 2 Marin Cilic. He conceded he didn't see this winning streak coming.

"I've just continued to keep playing better," he said. "I think today was even my best match. Physically I'm super fresh, and I'm ready to go for the final."

Alexander Zverev, of Germany, celebrates after wining a point against Pablo Carreno Busta, of Spain, during a semifinals tennis match at the Miami Open, Friday, March 30, 2018, in Key Biscayne, Fla. Zverev defeated Carreno Busta 7-6 (4), 6-2. (AP Photo/Wilfredo Lee)

Isner is seeking to become the first American man to win Key Biscayne since Andy Roddick in 2010.

"When he plays as good as today, he's one of the tougher guys on the tour," del Potro said. "He has everything to win a title like this."

Another American, reigning U.S. Open champion Sloane Stephens , will play No. 6-seeded Jelena Ostapenko of Latvia on Saturday for the final women's title on Key Biscayne. The tournament will move 18 miles north next year to the Miami Dolphins' stadium.

John Isner (14), United States, def. Juan Martin del Potro (5), Argentina, 6-1, 7-6 (2).

Alexander Zverev (4), Germany, def. Pablo Carreno Busta (16), Spain, 7-6 (4), 6-2.

Barbora Krejcikova and Katerina Siniakova (6), Czech Republic, def. Elise Mertens, Belgium, and Demi Schuurs, Netherlands, 4-6, 6-3, 11-9.

Ashleigh Barty, Australia, and CoCo Vandeweghe, United States, def. Ekaterina Makarova and Elena Vesnina (1), Russia, 3-6, 6-2, 10-7.