Peter Burling and Northland's Blair Tuke opened their 49er World title defence with two race wins and an 18th - on the first day of the 2015 49er and 49erFX World Championships.
The duo, who were last week named 2015 ISAF Rolex World Sailor of the Year, took a win in their first race of the day, but an 18th followed.
They came away from race three with another win to lie fifth overall in the early stages.
Talking last week at the 2015 South American Championships, Burling described the venue as tricky, and said the muddy-coloured water at the Buenos Aires venue made wind hard to read.
"It is a challenging place to get it right all the time so it's going to be a tough venue to sail the World s. It's rewarding when you get it right."