Great putt from Paul Casey on 14.

That was a horrible one from 20 feet but he judges that just right to find the heart of the cup. Nothing better than hearing your ball rattle in from distance.

-10 Mickelson

-6 Oosthuizen

-5 Koepka

-4 Streelman, Matsuyama, Grace

-3 DeChambeau, Bezuidenhout

-2 Hoffman, Casey, Im, Conners, Woodland

Selected others

E Spieth, Fowler, Finau, Fitzpatrick, Laird

+1 Lowry, Zalatoris, Poulter, Harrington

+3 Rahm, Hatton, Hovland

+4 MacIntyre, Rose, Westwood

+5 McIlroy

That's the first one he's missed all day... Phil Mickelson rushes through his birdie putt and it creeps left. He's down in par though, still five clear, but you'd have banked on the American making that with the form and momentum he has today.

Louis Oosthuizen makes no such mistake, drowning his birdie putt after reaching the green in there - he sits four back.

That Mickelson approach is a little further back than I initially thought, just down the slope leading the way to the dancefloor, and he clips it on beyond the pin to within five feet. Birdie chance incoming.

We've not had a DeChambeau update for a while because he's been trundling along nicely just stacking up par after par. I don't want to say boring, but for Bryson there's a distinct lack of drama.

Finally though he picks up a shot on the par-five 16th even though he pays two visits to the sandy waste areas, he still ends up on the green in three and has a 16-footer up the hill that he's able to give a good bash and rolls it in nicely.

Phil Mickelson has almost 250 yards up on to the front of the green. He reaches for a driving iron and crashes one almost exactly that distance, but it sticks on the fringes.

Not the eagle chance he hoped for but if he can make it down in two from here he'll stretch that lead even further.

Bogey for Grace after making a bit of a mess of the par five, with his third shot coming up short and spinning off the dancefloor then chunking his chip and missing his long par putt. Scrappy.

Koepka plays it better but is a bit too gung-ho with his birdie putt and leaves him a naughty little three-footer to test his nerves. He bangs it into the back of the hole though with authority.

Gosh. I didn't expect that. A poor third shot costs Hideki Matsuyama dearly but after several near misses that was sort of coming wasn't it?

The Masters champion finds the rough with his third and gives himself one of those nervy five-six footers for par which for once he doesn't sink.

Meanwhile, Christiaan Bezuidenhout is suddenly looking every inch the player who put himself in contention on Friday, arcing a wedge into three feet to give himself an easy birdie.

Must be a magnet on these fairways for Phil Mickelson today, another drive right down the middle at 11. He's making Kiawah Island look easy...

Louis Oosthuizen with a birdie chance of his own from the edge of the green, but it wheels away from the target at the last moment. He's left a few out there today...

Phil Mickelson stands over a seven-foot birdie putt that would take him to 10 under, fans stacked up on the bank behind in anticipation of him becoming the older major winner, and they can watch it all the way in from there! Mickelson is out to 10 under and leads by five!

Image source, Getty Images

Oh, Phil. He's here for the thrill.

Another delicious approach into the 10th, teeing up a birdie chance that would take him to 10 under.

Sublime iron play from the five-time major champion.

Just a quick note on Justin Rose.

The 2013 US Open champ is back in the clubhouse after a remarkable day. Ordinarily a scorecard containing eight birdies would be very welcome but he sadly also dropped nine shots in a round that included double and triple bogeys.

Rows of fans watching Brooks Koepka and Phil Mickelson in major action on the greens - isn't it great to see something almost like normality again!

Image source, Getty Images

Image source, Getty Images

Par. Par. But now Hideki Matsuyama is back in the rough off the tee on 11.

What does it say on the course guide here, 'the key to this hole is an accurate lay-up'. He could have made that a touch trickier with that loose shot.

Christiaan Bezuidenhout is about 15 yards inside him on the first cut, buoyed by a birdie on the last.

Image source, Getty Images

I'm not sure you can putt angry, but it had that feel about it as Koepka jammed in his birdie putt from 11 feet to get back to five under.

He's a even-keel kind of guy but he's made a couple of errors today resulting in bogeys that will annoy him.

Grace remains alongside him on five under as they chase the wonderful Phil Mickelson.

Louis Oosthuizen leaks his drive right at 10 and it skips across the sand before hitting a TV wire, which might just have stopped it wriggling towards the water...

Phil Mickelson thunders his drive straight down the middle. "It's between them where we want it..." he tells an onlooker who asks if it found a bunker.

Kevin Streelman! He's played that par five wonderfully well after hitting a laser from 266 yards up to 14 feet from the flag - and the eagle putt looks in all the way until it just hits the breaks inches before the cup.

It's agony for Streelman but he's still right in the mixer.

Great par-save for Mickelson after that sand shot a few moments ago and he maintains his four-shot lead at the top. The 50-year-old means business... four under for his front nine without a blemish.

Louis Oosthuizen saves par himself and remains level for the day at five under.

Paul Casey keeps threatening to join the party before being blown off course by an errant bogey.

The world number 20 has not really had too many decent chances today and now brilliantly skips his ball to three feet to simply collect a third straight par.

Meanwhile, further up the course Ian Poulter has almost recovered all the shots he shed between holes six-nine, with a birdie on the 15th.