World number two Carlos Alcaraz retired from his first-round match Tuesday at the ATP Rio Open after rolling his right ankle on just the second point of the contest against Thiago Monteiro.

The Spanish star took a medical timeout, had the ankle taped and won the first game but after dropping his serve in the second game he called it quits, in another setback to a so-far disappointing 2024 campaign.

The two-time Grand Slam winner had slumped to a four-set defeat by Alexander Zverev in the Australian Open quarter-finals.

Last week he fell to Nicolas Jarry in the semi-finals at Buenos Aires, and hasn't won an ATP title since his stunning Wimbledon triumph over Novak Djokovic last July.

Hopes that he could start turning things around in Rio -- where he lifted the trophy in 2022 -- were quickly dashed.

He and 117th-ranked Brazilian Monteiro were two points into the match when Alcaraz's right foot stuck in the red clay surface at Jockey Club Brasileiro, where rain had delayed the start of play for three hours.

Alcaraz took a hard fall and immediately asked for a medical timeout, limping to his chair and putting a towel over his face before the trainer arrived.

Alcaraz's coach Juan Carlos Ferrero looked on with concern from the stands as Alcaraz, with a grimace and a slight limp, returned to action and won the opening game.

But after dropping his serve in the second he opted not to continue.