By Jeb Blount

RIO DE JANEIRO (Reuters) - Tonci Stipanovic clinched Croatia's maiden sailing medal on Saturday as he stood first overall after the opening-round races in the men's Laser class, assuring him a gold or silver on points before Monday's final race.

Stipanovic, who can only be denied a gold by Australia's Tom Burton and can finish no worse than second, guaranteed his spot on the podium on a sunny day with moderate, lightly-shifting winds and light waves on Guanabara Bay.

"It's still sinking in, this first medal, this was the best day on all five days of racing," Stipanovic, who finished fourth at the 2012 London Olympics, told reporters. "I managed to pass Tom on the downwind, and now its between just him and me."

Despite only one first in the opening round, Stipanovic earned his spot at the front of the 10 medal-round finalists out of a 46-boat fleet with eight top-10 finishes in the preliminary races.

Stipanovic will win either gold or silver if he shows up for the medal race, World Sailing, the sport's governing body said.

Only Burton can catch Stipanovic for the gold in the medal race and New Zealand's Sam Meech, third overall, is Burton's only threat for silver.

Bronze is up for grabs, and is the only medal five-time Olympic sailing medalist Robert Scheidt of Brazil can win after following Friday's strong performance in wild waves with two weak finishes on Saturday.

Scheidt, an eight-time Laser world champion known as "El Demolidor," was second overall on Friday but slipped three places after finishing 26th and 11th in Saturday's races.

At 43, Scheidt, the oldest in the fleet, will have to turn in a near-perfect performance on Monday and hope rivals finish poorly to capture the unprecedented bronze.

In the Women's Laser Radial class, the medal race will decide the gold, silver and bronze with less than 20 points separating the top four finishers of the opening round.

Marti Bouwmeester of the Netherlands stands first with 47 points, Denmark's Anne-Marie Rindom is eight back with 55 and Ireland's Annalise Murphy is third with 66.

Belgium's Evi van Acker, the defending bronze medalist brought low by the lingering effects of a gastrointestinal illness her doctors suspect came from Guanabara Bay's polluted waters, bounced back to move into fourth overall.

(Editing by Frank Pingue)