Gauff spats with umpire as Olympics singles hopes end in tears
Gauff, the world number two, was one of the favourites to claim singles gold in Paris, but will instead have to try her luck in the doubles after crashing out in the third round.
The American lost 7-6 (9-7) 6-2 on Tuesday in a contest in which she clashed with chair umpire Jaume Campistol towards the end of the match.
A Vekic return had been called out by a line judge, with Gauff having not kept the ball in play, but Campistol believed the shot had landed in and handed the Croatian the point and a 4-2 lead.
Play was subsequently delayed as an emotional Gauff approached the umpire, saying: "I never argue these calls. But he called it out before I hit the ball.
"It's not even a perception; it's the rules. I always have to advocate for myself.
"It always happens here at [Roland-Garros] to me. Every time. This is like the fourth, fifth time it's happened this year."
Gauff felt she was on the harsh end of a decision in the semi-finals of the French Open earlier this year, in a defeat to Iga Swiatek.
Data Debrief: Vekic a worthy winner
Despite Gauff's complaints, she was not at her best, and Vekic deserved the victory.
Vekic struck 28 winners in total to Gauff's nine, as she claimed the first top-10 scalp of her career on clay.
It is the second time in the space of two Olympic Games that Vekic has ousted a top-three player, having defeated Aryna Sabalenka in Tokyo.