Until now.

The big-serving American is in the semifinals of the Miami Open for the second time in the past four years after overpowering South Korea's Hyeon Chung 6-1, 6-4 in just over an hour Wednesday afternoon. The 14th-seeded Isner finished with 13 aces, won all but one of his 32 first-serve points and avenged a loss to the 19th-seeded Chung at Auckland in his first match of the year in January.

"I played extremely well," said Isner, who dropped six of his first eight matches of 2018 before arriving in Miami and winning eight of his nine sets played at Key Biscayne so far.

"Every match I've played in this tournament, I've gotten better, I've gotten stronger, and that's a very, very good sign."

He'll hope that trend continues in the semifinals.

Isner -- who ousted second-seeded Marin Cilic earlier this week -- will next face No. 5 seed Juan Martin del Potro of Argentina, who rallied to beat No. 20 Milos Raonic of Canada 5-7, 7-6 (1), 7-6 (3) in the late match Wednesday in a slugfest that lasted nearly three hours.

Isner was broken twice by Chung when they met in Auckland. He faced only one breakpoint on Wednesday.

"It's perfect conditions," said Isner, who lost to Novak Djokovic in the 2015 semifinals at Key Biscayne. "I've played well here in the past, and I'm so happy that I'm playing well here again."