Defending champion John Isner hammered 12 aces to beat Izak van der Merwe 6-4 7-6 (7-2) to reach the semi-finals of the ATP Tour’s Hall of Fame Tennis Championships on Friday.

Top seeded Isner will square off against fellow American Ryan Harrison who was leading 6-4, 3-0 in his quarter-final when Benjamin Becker, of Germany, retired with an injury.

Isner, who beat Belgian Olivier Rochus in last year’s final, won 85 percent of his first-serve points against the South African qualifier on the grass courts.

Australian veteran Lleyton Hewitt will face American Rajeev Ram in Saturday’s other semi-final.

Hewitt is a two-time major championship winner and Ram won this event three years ago.