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."
A second kiwi crew, Markus Somerville and Aaron Hume-Merry, are also lying within the top 10 with three single-digit results (9, 5, 9) from the first day of competition.
Currently standing in the top three places in the 49er are Croatia's Kostov and Cupac leading, Spain's Paz Blanco and Paz in 2nd while Poland's Przybtek and Kolodzinski are 3rd.
The NZL Sailing Team's 49erFX pair of Alexandra Maloney and Molly Meech are lying 20th.
They sailed four races on Monday placing 12th, 11th, 4th and 9th.
The regatta, which has attracted 61 boats in the men's 49er fleet and 44 in the women's 49erFX fleet, had another five days of racing scheduled before medals were decided.
Racing resumed yesterday with the women racing first.