Blockx, who started the year ranked outside the top 100, said he was "close to retiring" because of sharp pain in an injured rib in the first set.
Alexander Blockx’s dream debut continues at the US Open
Alexander Blockx's dream debut continued with a gritty 6-3, 4-6, 7-5, 7-5 victory over Argentina's Francisco Cerundolo, which made the 21-year-old the first Belgian man to reach the US open last eight.
He next faces Karen Khachanov after the Russian beat 14th-seeded American Learner Tien 7-5, 4-6, 6-7 (6/8), 6-1, 6-4.
