It has been a surprisingly easy ride for Lajovic, who has yet to drop a set in the tournament. He earned back-to-back upsets, defeating No. 16-seeded David Goffin in the second round and No. 4-seeded Dominic Thiem in the round of 16.
"I'm very happy today. I didn't expect this, especially not in two sets against a player like Dominic," Lajovic said after his stunning win over Thiem.
Last year, the 28-year-old Serbian reached his first Masters 1000 quarterfinal in Madrid as a qualifier, beating Juan Martin del Potro along the way.
Next up for Lajovic is compatriot and top-seeded Novak Djokovic or Daniil Medvedev, who defeated No. 6-seeded Stefanos Tsitsipas for the fourth straight time.