Spieth and Koepka set the Open standard

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Jordan Spieth and Brooks Koepka each proved low scores were possible at Royal Birkdale by shooting 65 to share the lead midway through day one of The Open.

Wind and rain made life tough for those out earliest on Thursday and 1998 champion Mark O'Meara was among many to struggle, running up an eight at the first on his way to an 81.

However, conditions eased somewhat as the day wore on, allowing two-time major winner Spieth and last month's U.S. Open champion to set a demanding benchmark.

In his pre-tournament news conference, Spieth suggested around half of the field could effectively be ruled out of contention this week if they find themselves on the wrong side of the draw.

The 23-year-old is among those likely to face the sternest test on Friday afternoon, when gusts of over 30mph could potentially wreak havoc.

Yet Spieth will at least have a head start on the vast majority of his rivals after his bogey-free opening round, which could have been even better had he not missed a short birdie putt on the last.

The Texan did pick up shots at the second, eighth, ninth, 14th and 17th, while a superb up-and-down from the left of the 10th green provided another highlight.

Koepka charged to the top of the leaderboard with three birdies in a row from the 11th and followed a solitary bogey at 16 by holing a bunker shot for eagle at the next.

Matt Kuchar, another American, was four under through six holes and occupied sole possession of third.

Ian Poulter - the runner-up to Padraig Harrington at Birkdale in 2008 - and Justin Thomas each went round in 67, while there were 68s for Alex Noren, Hideki Matsuyama, Stuart Manley and James Hahn.

Playing in the company of Spieth, defending champion Henrik Stenson started with a one-under 69, while Thomas Pieters matched that score despite being three over at the turn.

The likes of Dustin Johnson, Rory McIlroy and Jon Rahm were among the afternoon starters in Southport.