Fluky winds, moderate currents and crowded upwind fleet racing challenged sailors on the Olympic regatta's opening day as Britain's Nick Dempsey and France's Charlie Picon took impressive leads in the men's and women's RS:X windsurf class.

Winds on the Escola Naval course were steady and to rising between 11 and 14 knots only to later shift direction and weaken in a day which also saw action in the men's Laser and women's Laser Radial single-person dinghy competitions.

In the tougher and more inconsistent later conditions, Croatia's Tonci Stepanovic and the Netherlands Marti Bouwmeester led their respective classes after defending gold medallist Xu Lijia was disqualified from the second Laser Radial race on Monday after a right-of-way protest.

Both Dempsey and Picon picked up two firsts and a second in their three RS:X races. Dempsey's strong starts allowed him to lead all three races from the beginning, taking the first two with barely a challenge. There are seven races and a medal round left.

"Today was absolutely perfect,'' Dempsey said. ''In the first two races the wind was pretty stable, and that's as good as it gets in Rio."

"In the third race we got hit by some wind bombs. Lots of people got caught out."

Picon didn't start well, but used dominant down-wind sailing to overtake her competitors from behind.

"I was reassured on my speed today,'' said Picon, the world number nine. ''I'm comfortable on this course."

The men's Laser and women's Laser Radial classes faced tougher and more inconsistent conditions than the RS:X on the Pao de Acucar course. Stipanovic leads the Laser with six points after finishing first and fifth in Monday's two races.

Argentina's Julio Alsogaray is second, also with six points, after fourth and second place finishes. Russia's Sergei Komissarov is third with 11 points after finishing second and ninth.

Stipanovic and others were challenged by a sharp wind shift and in-course mark change in the second race.

"In the first race there was more pressure on the right hand side of the start, but I went with my tactics people and went left," he said. "I tried that in the second race and it didn't work so good."

Bouwmeester leads the Laser Radial after Xu's second-race disqualification with 10 points after a sixth- and a fourth-place finish. Ann-Marie Rindom of Denmark is in second with 13 points after finishing fifth and a eighth, and Ireland's Annalise Murphy is third with 14 points after a first and 13th.

Xu's disqualification, which added 38 points to her total, pushed her down to 23rd out of 37 boats. She finished third in the first race and now has 41 points.

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