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.

The 33-year-old 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.

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.

John Isner serves to Felix Auger-Aliassime, of Canada, during their semifinal match at the Miami Open tennis tournament, Friday, March 29, 2019, in Miami Gardens, Fla. (AP Photo/Lynne Sladky)
John Isner serves to Felix Auger-Aliassime, of Canada, during their semifinal match at the Miami Open tennis tournament, Friday, March 29, 2019, in Miami Gardens, Fla. (AP Photo/Lynne Sladky)

Auger-Aliassime was the youngest male semifinalist in the tournament’s 35-year history.

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Felix Auger-Aliassime, of Canada, returns to John Isner during their semifinal match at the Miami Open tennis tournament, Friday, March 29, 2019, in Miami Gardens, Fla. (AP Photo/Lynne Sladky)
Felix Auger-Aliassime, of Canada, returns to John Isner during their semifinal match at the Miami Open tennis tournament, Friday, March 29, 2019, in Miami Gardens, Fla. (AP Photo/Lynne Sladky)