Oh my. Hideki Matsuyama's challenge looks over.

He'd have just been hoping to have found the putting surface there with possibly a three iron but instead he flies it right through the back.

That's a big, big error at this stage and one that sees him drop back even further after failing to get close with his second. He looks pretty perplexed with the way things are unfolding right now.

As it turned out, Koepka's 10-footer for birdie was actually for a share of the lead, but he can't convert and has to settle for par. That's a killer as it would've been a huge momentum shift.

Grace makes back-to-back bogeys after being too aggressive with his tee shot and ending up off the back down a mountain of a slope.

Louis Oosthuizen leaves his par putt short and will make bogey, dropping back to six under.

Now can Phil Mickelson follow suit and escape with just the one dropped shot, coming down the slope, it'll just need a tickle from eight feet. Agonisingly wide to the left.

He drops two shots and the lead is cut to one.

Louis Oosthuizen gets a bit of a break with a drop, his ball must have skirted the land there before entering the water... he pops it up on to the green from there, well played.

Phil Mickelson, after three off the tee, crunches his approach on to the smooth stuff and this could be an excellent piece of damage limitation coming up. A presentable putt for bogey.

It's a bogey as expected for Grace but that's the best he could do after finding the drink.

Par for Brooks who then fires in a laser of an iron on the par-three 14th to set up a birdie chance and with the leaders in trouble this could get a lot tighter in a few minutes.

Ian Poulter's round has also come to a close. The Englishman had a birdie chance up 18 but saw his ball pull up just shy of the cup.

He is now tied for 24th after finishing one over today, meaning he's also one over for the championship.

Hang on, hang on a minute. On the face of it Bryson's a bit too far back on two under, but that birdie on 18 showed great character after his double at 17.

And more importantly the leaders are having a spot of water trouble and still have the hardest part of the course to navigate.

This isn't over just yet.

Oh wow.

Louis Oosthuizen leaks his tee shot at 13 into the drink, seemingly handing Phil Mickelson a huge advantage. But then Lefty follows him into the water with his. Remarkable.

The American has to reload, and finds the fairway this time. But this is all getting very nervy.

Disaster for Grace as he dumps his second shot in the water from 196 yards out - huge error there and he'll need to scramble now to limit the damage.

It seems that bogey we mentioned was just a pipe dream for Hideki Matsuyama on the 12th.

His bogey putt slides left. That was never turning back played at that pace. The Masters champion is in a really rocky spell at the moment. Eight shots back now. He needs a big turnaround.

Two bogeys on the spin and now Hideki Matsuyama is in trouble on the 13th as he finds himself in a thick clump of wispy grass set back about 15-20 yards from the green.

Can he magic something up from there? Nope. His ball scuttles out but rests on the fringe about 40 feet from the cup. Surely that means a bogey at best.

I tell you what, that is almost magnificent from Phil Mickelson. 27 feet from the cup, he catches the line and rolls it almost all the way in, just coming to a halt a few rotations short. Bogey it is, his first of the day.

And moments later, Louis Oosthuizen skips in a birdie putt to make it a two-shot swing on the 12th.

The South African climbs to seven under, as Lefty slips to nine under.

Mickelson finds a sprinkler head on the fairway and shouts across to Louis Oosthuizen to check he's watching him take a free drop.

He then flicks a wedge on the left edge of the green, but this'll be a tricky par putt.

Oh dear... No sooner had DeChambeau picked up a shot then he hands two right back on the dreaded 17th as his tee shot leaks right and finds the water.

A double is always on the cards from then on in and it's a five that drops him to one under and you've got to think he's way out of it now.

Image source, Getty Images

Bogey. Bogey. Hideki Matsuyama could not have picked a worse time as the leaderboard starts to look all congested again.

He slips back to three under after three putting. He can't even blame his tee shot there. He was in perfect position and on the dance floor in two.

Christiaan Bezuidenhout also sees a shot disappear after tugging his tee shot left and having to take his medicine, pitching back on to the fairway.

Well, a missed birdie putt at 11, and now he's found a bunker off the tee at 12, that they are two of the loosest shots in Phil Mickelson's round today tells you all you need to know.

This could be an awkward lie for the left-hander though, tucked on the bottom of the right bank. A very awkward stance, and he'll have to settle for laying it up on to the fairway.

Lovely 20-footer for birdie as Koepka just starts to get things rolling now and moves to six under, still four back but looking a lot more like it.

Shots can come and go in a hurry down this closing stretch too.

This is definitely a good run from Phil Mickelson...