American Alex Meyer and Italy's Rachele Bruni were winners at the first leg of this year's International Swimming Federation (FINA) Marathon Swimming World Cup in Patagones-Viedma.
Meyer, the 27-year-old World Championships silver medallist in the 25 kilometres, won in the 10km event after coming home ahead of Italian Simone Ruffini at the Argentinian venue.
This was a role reversal from the World Championship race in Kazan last year, when Ruffini took the gold medal, but the winning margin was less than a second as the pair were neck and neck.
The American finished in 1 hour 55.44 min, with another Italian, Federico Vanelli, coming home third.
Germany’s Christian Reichert, the overall winner in 2015, could only manage a 15th place finish.
Meanwhile, women's overall champion Bruni got her title defence off to the perfect start with a winning time of 2:06:11.
She won by more than three seconds ahead of Brazil's Poliana Okimoto, while Ecuador's Samantha Arevalo completed the podium.
In all 38 men and 24 women entered the race, with six more events scheduled this year.
The World Cup will now move to Abu Dhabi in the United Arab Emirates on February 26.
There will then be a break before the third event of the season on June 18 in Balatonfured, Hungary.
This will see swimmers take to Lake Balaton - a venue which will be used if Budapest is successful in its bid for the 2024 Summer Olympics.