SOUTHPORT - Jordan Spieth produced one of his best major championship efforts and Brooks Koepka underlined his credentials as the American duo both shot first-round 65s to share the British Open clubhouse lead on five under par.

Spieth, the world number three, avoided the grim early-morning weather at Royal Birkdale on Thursday as he went out with reigning champion Henrik Stenson and promptly fired a flawless round featuring five birdies and not a single dropped shot. "I couldn't have done much better today," admitted the Texan, even if he agonisingly failed to convert a putt for birdie at 18. "Everything was strong. I thought I'd give it a nine across the board for everything -- tee balls, ball-striking, short game and putting. So things are in check. It's just about keeping it consistent."

Winner of both the Masters and US Open in 2015, Spieth had failed to break par in his last five opening rounds at a major before coming to the north-west English coast this week. He is now hoping to have set himself up to survive what are expected to be gruelling wet and windy conditions on Friday, although plenty other leading players will be thinking likewise.

Koepka, fresh from a month off after winning his first major at the recent US Open, was one-under at the turn before a rollercoaster ride on the way home. That saw the 27-year-old birdie three holes in a row from the 11th, bogey 16 and then eagle the par-five 17th in sensational fashion as he holed out from a bunker.

"I played really solid. And unfortunately one bogey on 16, just a bad putt," Koepka said. "But 17 was actually a terrible lie in the bunker. It was in one of the those rake marks. And my caddie told me to get inside 10 feet, that would be pretty good. And luckily enough it went in."

A victory for either man this week would end the recent run of seven consecutive majors going to first-time major winners. But plenty potential new champions are putting pressure on the leading duo. Royal Birkdale again brought the best out in Ian Poulter as the Englishman fired a three-under-par 67.

It was a fine return to the big stage for Poulter, who had not appeared at a major since the Masters last year and had to settle for a role as a television commentator at the Open 12 months ago while sidelined through injury. He came through qualifying at his home course Woburn to make it to Birkdale, where he finished runner-up to Padraig Harrington in 2008 in what remains his best performance at a major.

Justin Thomas, an emerging American star and the current world number 13, also shot 67, his round including three birdies and an eagle at the 17th, which is one of just two par-5s on the Birkdale set-up. Thomas looked the part in an outfit featuring a shirt, tie and cardigan, and said: "Obviously I knew it was going to get a lot of publicity out there. But I didn't come here to dress well. I came here to try to play some good golf. And I guess that just happened."

England's Richard Bland -- appearing at his first Open since Birkdale in 1998 -- also shot 67 after ending his round in stunning fashion with three consecutive birdies, while a host of players went round in 68, two under par. Among them were the Swedish world number nine Alex Noren, Welsh 38-year-old major championship debutant Stuart Manley, and perhaps most ominous of all the Japanese world number two Hideki Matsuyama.

Stenson, looking to become the first man since Harrington nine years ago to retain the Claret Jug, was one under par. World number one Dustin Johnson and 2014 Open champion Rory McIlroy were among those starting their rounds later in the day.