[1] On 11 July 2016, Zekic reached his best singles ranking of world No.

Zekić made his ATP main draw debut in doubles with compatriot David Savić as a wildcard pair at the 2011 Serbia Open, losing in the first round to top-seeded pair Zimonjić/Granollers.

In July 2016 he won his first trophy on the ATP Challenger Tour winning Internazionali di Tennis dell'Umbria in Todi.

Zekić made his ATP main draw debut in singles as a qualifier at 2017 Croatia Open Umag, losing to Radu Albot in the first round.

In two weeks' time, he played in another ATP tournament, 2017 Generali Open Kitzbühel, defeating top seed Alessandro Giannessi and eighth seed Stefanos Tsitsipas in the qualifying rounds, and earned his first ATP main draw victory by defeating world No.