SOUTHPORT, England — This first round of the 146th British Open was less about weather Thursday — although there was a wee bit of rain and considerable wind — than it was about names, big names.

Three of the biggest, Americans Jordan Spieth, Brooks Koepka and Matt Kuchar, each shot a 5-under-par 65 at Royal Birkdale to top an impressive leaderboard.

Spieth is the 2015 Masters and U.S. Open champion; Koepka is the U.S. Open champion from a month ago, and Kuchar, if without a major in his career, is a winner of the World Match Play and The Players, the almost-major.

A shot behind at 66 are Charl Schwartzel, the South African who won the Masters in 2011, and Englishman Paul Casey, who lives in Scottsdale, Arizona. Seemingly everybody is at 67, including Ian Poulter, the Englishman who lives in Florida and whose dream since a club pro in this country has been to win the Open.

Kuchar said not to get too excited for a while. “I always think you can judge a lot about a golf course by its leaderboard Sunday afternoon. It’s Thursday. So we’ll wait until Sunday and see what the leaderboard looks like.”

He had to like the way it looked early. “To start my British Open with a 29 on the front nine,” Kuchar said about going 6 under at the turn, “is a great way to start.”

And according to Spieth a great start was a necessity, because Friday’s second round is supposed to be pummeled by bad weather. “I thought today’s round was extremely important, as they all are,” Spieth said. “But given the forecast coming in, I thought you really needed to be in the red [under par] today.”

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Thirty-nine players were in red, as contrasted to the last Open at Birkdale, 2008, when the weather was nasty and only three broke par the first round.

Spieth knows the vagaries of Open conditions. He was trying for the unprecedented sweep of the Masters, U.S and British Opens two years ago and got derailed by storms at St. Andrews, though he finished a shot out of a playoff there.

“But today,” he said of a bogey-less round, “everything was strong. I give it a nine [out of 10] across the board, tee balls, ball-striking, short game and putting.”

Spieth is coming off a vacation in Los Cabos, Mexico, where he ended up in a group with Michael Phelps, Michael Jordan, Russell Wilson, Dwight Freeney and Fred Couples.

Koepka was away from golf after his U.S. Open win. “I’m mentally recharged,” he said.

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He had only one bogey, on the 16th, but followed that by holing a shot out of a bunker on 17 for an eagle. “Any time I can come over and play,” said Koepka, “I’m looking forward to it and I’m up for the challenge.”

Rory McIlroy seemed headed to a big score after making bogey on five of the first six holes but birdied three of the last four holes for a 1-over 71.

Mark O’Meara, the 1998 Open champion here and the first player off in the first round, hit into the rough, took a double-bogey six and shot 81.

“To have my name on the claret jug,” said O’Meara, 60, “is an amazing thing.”

Those big names on the board are hoping to find out how amazing.