The 6-foot-11 Karlovic ousted Samuel Groth of Australia 6-4, 6-4 on Saturday. Hewitt, who lost in the 2013 final to Nicolas Mahut, beat seventh-seeded American Jack Sock 6-1, 6-2.
The matches were played after the induction ceremonies for a five-person class for the International Tennis Hall of Fame, which was highlighted by Lindsay Davenport.
Davenport entered the hall with five-time Paralympic medalist Chantal Vandierendonck in the recent player category. Coach Nick Bollettieri, executive Jane Brown Grimes and broadcaster John Barrett were inducted from the contributor category during a 90-minute ceremony on Newport's grass courts.