The 20-year-old was quickest around the 10 kilometres course, posting a time of 1 hour 29min 50sec at the Parque Urbano de Albarquel.

"I’m really surprised as I’m in the middle of my training sessions for the World Championships at home, in four weeks at Balatonfured," said Rasovszky.

"Swimming in a very fast pace due to strong currents.

"But I had today a good performance and could keep a good speed in the last lap."

Second was Germany's Rob Muffels, the silver medallist in the 5km event at the 2015 World Aquatics Championships in Kazan, in 1:29.52

Italy’s Andrea Manzi secured a maiden World Cup podium finish as he took the bronze medal with a time of 1:29.59.

In the women's event, Italy’s Rachele Bruni claimed her eighth victory on the World Cup circuit since 2015.

Bruni took the Olympic silver medal at the Rio 2016 but finished in first place today in 1:37.36.

"I’m happy with the result and it was a good race," said Bruni.

Two South Americans completed the podium.

Brazil's Viviane Jungblut took second place, just behind Bruni in 1:37.37, while Ana Marcela Cunha of Ecuador secured the bronze medal in 1:37.42.

The next stage of the World Cup is due to take place in Lac Saint-Jean in Canada on July 27.