The four-time major champion mixed eight birdies with a lone bogey to card a seven-under 65 and top the leaderboard alongside Jon Rahm and Viktor Hovland, but Koepka was questioned post-round over an incident at the par-five 15th.

Koepka was four under for his round when he reached the 550-yard hole, where the former world No 1 hit the green in two to set up a two-putt birdie, with TV cameras appearing to show his caddie - Ricky Elliott - revealing the club selection to a playing competitor.

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Elliott twice looks to be mouthing "five" to Gary Woodland's caddie Brennan Little, indicating that Koepka used a five-iron for his approach shot, with social media suspecting that a rules violation had been made.

Why was the incident controversial?

According to Rule 10-2a in the rules of golf, a player must not give advice to anyone in the competition who is playing on the course, ask anyone for advice - other than their own caddie - or touch another player's equipment to learn informational that would be advice if given.

The ruling would also apply for information provided by a caddie towards an opponent and the penalty for a breach would be two strokes, although tournament officials reviewed the incident and decided there had been no breach.

"Following the completion of Brooks Koepka's round, the Committee questioned his caddie and others in the group about a possible incident on No 15," Augusta National said in a statement.

"All involved were adamant that no advice was given or requested. Consequently, the Committee determined that there was no breach of the Rules."

What did Koepka say?

Koepka denied any wrongdoing when speaking to reporters after his round, having reviewed the incident with Augusta officials.

"Yeah, we looked at it when we got back in," Koepka said. "GW [Woodland] and Butchie [Little] had no idea what we were hitting.

"They didn't even know because - I know that fact, because GW asked me what we hit walking off, when we were walking down. So that's all I can give you."

Was Koepka lucky to avoid being penalised?

The ruling was discussed at length ahead of the second round at Augusta National, with former players and experts questioning the verdict from a situation where a breach was "very obvious".

"I think 'appear' to say five would be soft - he does say five!" Brad Faxon told Sky Sports News. "This happens a lot, where caddies talk to each other. It's done, it's not really enforced, and there's a looseness to it. It should be enforced, because it is a rule of golf, it's about protecting the field.

"If this happened on Sunday, and they were in the final group or close to the lead, would it be explained differently? Yes, it would be. They were caught red-handed. On this particular rule, it could have been handled better. Brooks is a lucky boy."

Ryder Cup-winning captain Paul McGinley, speaking to Golf Channel, added: "It's staggering that they denied it because the video evidence is there. This is common practice on tour, whether you like it or not and happens in every professional tournament around the world. It's not always so blatant."

The last 17 Masters champions were inside the top-11 after the opening round of their Green Jacket successes, with Koepka now having a chance to extend that streak and earn a fifth major victory.

Koepka was already a big storyline ahead of the opening major of the year as one of the 18-strong LIV contingent in the Augusta field, with this opening-round incident likely to be revisited should he end up winning the tournament.

"The ruling has been made and that's that," Ryder Cup captain Luke Donald told Sky Sports. "It did seem quite clear that there was some information being given, but it does happen. Certainly, I don't give out what I'm hitting on par-threes but I wouldn't hide what club I'm hitting, but you've got to take the word of Brooks [Koepka] and Ricky [Elliott]."

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