Right then, there are a couple of Brits scaling the heady heights of the leaderboard early on.

England's Tommy Fleetwood is already two under par having played four holes - he started at hole 10.

Scotland's Martin Laird, meanwhile, has carded back-to-back birdies on his opening two holes.

The first of those came at the 613-yard par-five opening hole.

On Tuesday, on an already wet course with more rain predicted, organisers took the decision to cut fescue from holes four, 12, 14 and 18 at Erin Hills.

The knee-length grass was already dividing opinion among the world's top players, with several critiscing the depth and thickness of the rough.

Others, namely Rory McIlroy and Jordan Spieth, said that US Opens were supposed to be tough.

"If we can't hit within that avenue, you might as well pack your bags and go home," said the Northern Irishman.

Luckily, BBC Radio 5 live's Chris Latchem has put together this handy video to answer all you fescue-related questions.

Now back to the golf...

There's the chance of some rubbish weather, for one...

Image source, Getty Images

Which some claim will make the fescue unplayable...

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And then there's whether it should stay or go...

Image source, Getty Images

OK, you're going to be hearing a lot about 'fescue' this week...

So if, like me, you weren't aware of the knee-high grass before this week's US Open, then here's a quick definition.

Fescue: "any grass of the genus Festuca: widely cultivated as pasture and lawn grasses, having stiff narrow leaves"

So, we join you just as a group containing three former winners gets under way.

Dustin Johnson is looking to win back-to-back US Opens and the world number one goes out alongside Martin Kaymer and Jordan Spieth.

Here's who is up in the next half an hour:

14:35 BST: Ernie Els, Webb Simpson, Lucas Glover

14:35 (hole 10): Martin Kaymer, Jordan Spieth, Dustin Johnson

1446: Tyson Alexander, Christopher Crawford (a), Max Greyserman

14:46 (hole 10): Joaquin Niemann (a) (Chi), Joel Stalter, Stephan Jaeger

1457: Matthew Campbell, Garrett Osborn, Lee Walker (a)

14:57 (hole 10): Daniel Miernicki, Sahith Theegala (a), Tyler Light

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Mexico's Roberto Diaz takes Phil Mickelson's place and will tee off in a group with Stewart Cink and Steve Stricker at 20:20 BST.

World number 23 Mickelson was hoping rain would delay the start of Thursday's round and allow him to be at daughter Amanda's graduation ceremony and then fly by private jet to Wisconsin.

Amanda was born a day after the US Open at Pinehurst in 1999, when her father finished second to Payne Stewart.

The American said he would have left, wherever he was on the scoreboard, had wife Amy gone into labour.

Image source, Getty Images

First to that news on Mickelson, who has withdrawn from this year's US Open to attend his daughter's graduation.

The five-time major winner had a tee time for the tournament but, with the first round at Erin Hills coinciding with daughter Amanda's graduation in California, was relying on some poor weather to delay play and allow him time to return.

The American told the New York Times in the build-up to the tournament: "As I look back on life, this is a moment I'll always cherish and be glad I was present."

The US Open is the only major 46-year-old Mickelson has failed to win.

Image source, Getty Images

Ahhh, the opening day excitement of a major.

Glacier-carved rolling hills and pristine Wisconsin countryside diced up into 18 links-style holes with fairways skirted by... fescue.

Yes, the US Open may be upon us, but it's the knee-high grass forming the Erin Hills rough that's caused plenty of debate heading into Thursday's opening round.

World number one Dustin Johnson is there, five-time major winner Phil Mickelson isn't, while some of the world's finest are already out battling their way around what is the longest course in the championship's history.

We'll be keeping you up to date all weekend.