Isner, the defending champion, beat the 18-year-old Canadian 7-6, 7-6 to reach the final. He'll face either Roger Federer or Denis Shapovalov.
MIAMI GARDENS, Fla. — Defending champion John Isner earned a return trip to the Miami Open final by hitting 21 aces and defeating 18-year-old Canadian qualifier Felix Auger-Aliassime 7-6 (3), 7-6 (4) on Friday.
Isner won his only ATP Masters title a year ago. His opponent Sunday will be the winner of the second semifinal between three-time champion Roger Federer and another Canadian teenager, 19-year-old Denis Shapovalov.
''It's amazing,'' the 33-year-old Isner told the crowd after his victory. ''I said before the tournament the chances were I would not defend my title. I'm very, very happy.''
Auger-Aliassime served for both sets, but Isner broke each time by taking advantage of double-faults, and used his big serve to control the tiebreakers. Nine of the 10 sets Isner has played in the tournament have gone to a tiebreaker, and he has won each one.
Auger-Aliassime had been 5-0 previously against Top 20 opponents, and was the youngest male semifinalist in the tournament's 35-year history. He'll jump to a career-best No. 33 in next week's rankings.
But he'll need steadier nerves to fulfill his potential as a future Grand Slam champion.
Isner lost serve for only the third time in the tournament to fall behind early. Auger-Aliassime served at 5-4 but double-faulted into the net three times, including on break point.
There was a repeat in the second set. Auger-Aliassime served at 5-3, double-faulted once and hit a swinging volley into the net on break point.
Each lapse led to a tiebreaker, and Isner has been unbeatable of late in such situations, after arriving at Miami with a 7-10 record in tiebreakers this year. He finished strong against Auger-Aliassime, whacking three aces in the final tiebreaker, the last on a 140 mph serve that was his fastest of the match.
''I'm playing well,'' Isner said. ''Keep it up and see what happens.''
The women's final Saturday will match No. 5 Karolina Pliskova of the Czech Republic against No. 12 Ashleigh Barty of Australia