Roger Federer saved seven match points before fending off 100th-ranked Tennys Sandgren in five sets to reach the Australian Open semifinals for the 15th time.
Federer, who has won six Australian championships among his 20 major titles, took an off-court medical timeout in the third set and was on the verge of a shocking quarterfinal exit in the fourth before rallying to win 6-3, 2-6, 2-6, 7-6 (8) 6-3.
The 38-year-old Federer will next play seven-time Australian Open champion Novak Djokovic.