Jordan Spieth after his birdie at the 18th

Jordan Spieth gave Rory McIlroy a lesson in course management as he left the Co Down man trailing him by nine shots entering the final round of The Open.

The 23-year old from Dallas hardly put a foot wrong in terms of his strategy, rolling in a 20 footer for a birdie at the 18th for a 65 and a three-stroke lead on 11-under par from Matt Kuchar, who was hot on his heels until he double bogeyed the 16th and posted a 66 to sit alone in second on eight-under,

McIlroy felt his lack of trust in his swing derailed his hopes of winning The Open for the second time in four years. And with Spieth looking infallible up ahead, his frustration led him to making a poor and ultimately costly course management decision on the 10th hole.

After getting off to a fast start by making three birdies in his first six holes, the four-time major winner played the remainder in two over as he bogeyed the seventh and eighth, then hit a three-iron tee shot into a bunker at the 10th and barely escaped.

His recovery finished on the right lip of the bunker ahead and forced to stand in the sand with the ball well about him, he pulled his third left of the green and missed a 12 footer for bogey.

A double bogey six there killed all his momentum and he eventually limped to a 69 — the scoring average for the day was a remarkable 69.026 — to finish on two-under par.

"It definitely hurt," McIlroy said of the costly mid-round wobble that was only saved by a birdie on the ninth. "Bad swing on seven and I was just was a bit sloppy on the eighth hole.

"And then I just completely took the wrong club on the 10th off the tee.

"You either hit a club that stays short of all those bunkers or you take a club that at least only brings the traps up at 300 into play and I did neither."

Spieth, on the other hand, made sure he stayed out of the sand.

"A day like today, these bunkers a lot of time you're flying one but you're trying to keep it short of the next, which is very difficult," he said. "And luckily the fairways are softer and I thought it was easier to do that, especially without wind, to trust your shot and know that it's not going to roll too far, which made it a lot easier off the tee.

"But after a couple well-struck irons, I knew that as long as I put myself in the fairway or right near it, no matter how far back, I was going to have still have a birdie [chance]. But we still played aggressive today.

McIlroy picked up just one shot in his last eight holes and confessed that the missed six footer for eagle at the 15th and the poor drive at the 17th summed up his day.

But he also explained that it was a simple lack of trust in his swing that left him without the tools to go low on a day when 30 players shot 68 or better.

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"It's not quite where I need it to be to win the biggest golf tournaments in the world," he said of his swing. "But it's getting there."

That explained his failure to push on when he briefly got to within two shots of Spieth after five holes.

"I've always been good when I get off to fast starts being able to keep it going, and I didn't today," he said. "And I needed to, that's the disappointing thing.

"It's hard to think big picture now, I'm just off the golf course and I'm a little disappointed.

"This week has been a step in the right direction, there's no doubt about it. And I need to pick myself up, play a good round tomorrow and hope for some bad weather, and hope for some guys to struggle.

"I definitely feel like today was an opportunity lost to get right in the mix going into tomorrow."

Spieth shot a bogey-free, five-under par 65 to lead by three strokes on 11-under par from compatriot Matt Kuchar, who shot a 66 with 20-year old Canadian Austin Connelly and US Open champion Brooks Koepka six shots off the pace on five-under.

After the fireworks of Branden Grace's record-setting 62 in the morning, McIlroy looked set to threaten that number when he birdied the first from four feet.

He had that McIlroy strut of old again but within two hours it was clear that his game was not quite as razor sharp as it needed to be to keep pace with an impressive Spieth.

While he chipped in for an unlikely two at par-three fourth to get within three shots of the American on three under par, he had that steely eyed look of old.

The gap was two when he unleashed a 310-yard three-wood to 18 feet the driveable fifth, but unlike Spieth, whose putts almost always appear to threaten the hole, McIlroy's eagle attempt stopped in the jaws.

The tap-in birdie left him tied for third and just two behind but as Spieth got into his round, McIlroy began to struggle.

Shortly after the American fired a 172-yard approach to 12 inches at the third to get to seven under, McIlroy tugged his tee shot into the rough at the 156-yard seventh, fluffed his first chip and made bogey to slip back to three under.

Another shot went at the eighth where he missed the fairway and then chipped 12 feet past, leaving his par putt short.

While he rolled home a 12 footer for a three at the ninth to turn in two under 32, his title bid suffered a serious blow at the 10th.

Electing to hit a three-iron off the tee, he drove into sand and eventually took six after missing the green with his third.

He was soon eight shots behind Spieth who birdied the seventh and eighth to turn in nine-under par, two ahead of playing partner Kuchar.

It was a killer blow to the 28-year old from Co Down, and while had a chance to erase that 10th double bogey when he hit a sky-high four-iron to seven feet at the par-five 15th, he missed the eagle putt.

Jordan Spieth celebrates his birdie at the 15th

Spieth's efficiency on the greens can be intimidating, and while Kuchar briefly tied for the lead on nine-under with a two-putt birdie at the 15th, Spieth kept his lead when he made a ten-foot return putt for birdie.

"I thought the one on 15 was really big," Spieth said. "Because he tied the lead there. I'd hit a good shot in there, and hit what I thought was a good putt and it ended up turning into not so good of a putt. And that putt was very difficult, because with any speed getting to the hole, the hole was on a crown, it was going to go three or four feet by, and then I've got a three or four- footer for par for my putt.

"It was a scary one, probably the scariest of the day. And to knock that one in was the one that I thought was big to stay ahead."

Any hope McIlroy had of getting within touching distance of the leaders ended at the par-five 17th where he drove into the rough, and hit an indifferent third to 18 feet and failed to make the putt.

Spieth extended his lead to three when Kuchar double bogeyed the 16th and while the Texan failed to birdie the 17th, he twisted the knife on the 18th by holing a 20 footer for birdie and a three-shot lead as he chases the third leg of the career grand slam today.

His strategy will be key and he feels that with slightly tougher scoring conditions, he will be able to play more conservatively and win.

"I think the conditions are going to help me pick and choose," he said. "I feel very confident in the way I've been striking my irons. If that continues and I feel that way as we get in our warm-up and then early in the round tomorrow, then I can play more conservative off the tee and take a lot of those bunkers out of play.

"And that's obviously very important. Greens in regulation I hit, again, probably 16 today, like I did the first round. Probably not realistic tomorrow, but I think it will be a bit easier to kind of figure that out.

I think it will be easier than today to get out there and play a bit safer because I don't think it will be as scoreable."