Australia's Mat Belcher and Will Ryan struggled on the opening day of competition in San Isidro ©Getty Images

Defending champions Mat Belcher and Will Ryan endured a difficult start to their title defence at the 470 World Championships in San Isidro, Argentina.

The Australian duo have won gold at the past three editions of the event, with their latest triumph coming in October.

However, they now face a battle to make it four titles in a row after being disqualified in the opening race of the regatta, before finishing 21st in the second.

The results leave them in 33rd position in the rankings on a total of 64 points.

New Zealand’s Paul Snow-Hansen and Daniel Willcox made the most of their rival’s problems by securing wins in both races to lead the standings on two points, with 2015 silver medallists Sime Fantela and Igor Marenic occupying second position.

The Croatian duo recorded fifth and sixth place finishes to begin their week with a total of 11 points, as they aim to end their wait for a first world title since 2009.

"That's how we wanted to start," said Snow-Hansen, following the two race wins.

The United States' Annie Gaegar Briana Provancha lead the women's standings
The United States' Annie Gaegar and Briana Provancha lead the women's standings ©Getty Images

"We have put a lot of time in during our off season at home and we've been looking forward to Argentina and it is good to finally be here.

“We are really, really happy to have a good first day, but there's a long way to go yet.

“When Paul and I first teamed up in 2013 we had a really good opening series at that year's 470 Worlds, so yes we have had similar days, but nothing quite like this."

In the women’s event, the United States’ Annie Haegar and Briana Provancha lead the standings on five points after securing second and third place finishes.

China’s Xiaoli Wang and Lizhu Huang are just a point further adrift after backing up finishing as the runner-up in the opening race of the day by finishing fourth in the second.

A win for the United States’ Sydney Bolger and Carly Shevitz had given them the perfect start to the regatta, but they ended the day in third position after crossing the line in 11th place in the second race.

French duo Camille Lecointre and Helene Defrance won race two but failed to finish the first and are 20th overall.

The regatta is due to take place until February 27.