Louis Oosthuizen will take a one-shot lead into the final round of The 149th Open as Jordan Spieth endured a disappointing finish to day three at Royal St George's.
Oosthuizen overcame a mid-round stutter to fire a creditable 69 as the superb scoring of the previous two days was countered by a tough course set-up for the third round in some of the most benign conditions at any Open in recent memory.
Having slept on a two-shot lead overnight after becoming the first player in Open history to shoot 129 over the first 36 holes, Oosthuizen did just enough to retain his position on top of the leaderboard ahead of debutant Collin Morikawa.
Spieth was four under for the day after 11 holes and shared the lead with Oosthuizen and Morikawa with three holes remaining, but the 2017 champion bogeyed the 17th and missed a short putt for par on the last to slip three strokes off the pace.
Oosthuizen's overnight lead remained intact as he parred the first six holes, with Morikawa dropping four behind with bogeys at the second and fifth, leaving Spieth to apply the front-nine pressure on the South African.
Spieth birdied four of the first seven holes to close within one shot of Oosthuizen, who responded with his first birdie of the day at the seventh before he added another at the ninth moments before the American converted an excellent approach to the 10th to get to 12 under.
But Spieth and Oosthuizen both dropped shots after errant approaches to the 11th, and the 2017 champion scrambled a valuable par at 12 after his gouge from the left rough bounded over the back of the green.
Spieth then found himself in a share of the lead when Oosthuizen dropped another shot at the 13th, where Morikawa birdied and added another at the long 14th to make it a three-way tie at the top of the leaderboard.
Oosthuizen was unable to follow his playing-partner in, and he avoided another blemish when he held his nerve over a tricky six-footer for par on 15, and the successful putt inspired him to fire a short-iron in close at the 16th.
The 2010 champion made no mistake with the putt and returned to 12 under just as Spieth made a mess of the 17th and then three-putted from inside 15 feet at the last, missing his par putt from barely two feet in a sour finish to his 69.
Oosthuizen did well to get down in two from the front of the 18th green, and he was guaranteed the outright lead when Morikawa's 12-foot birdie putt at the last drifted left of the target, the young American tapping in to complete an impressive inward 33 and a round of 68.
There was little attention on Corey Conners after he parred every hole on the front-nine to remain stuck on four under, but the Canadian produced a sparkling run of birdies after the turn to propel himself into contention for a maiden major title.
Conners began the inward run with four birdies in five holes, and four closing pars added up to a 68 which left him just four shots off the lead along with Scottie Scheffler after the WGC-Match Play runner-up made only his second birdie of the round at the last for a 69 featuring 15 pars.
Jon Rahm's quest for a second consecutive major victory looked done and dusted when his third bogey of the day at the 11th dropped him to four under and nine strokes behind Oosthuizen at the time.
But as the leaders began to falter, the US Open champion responded with birdies at the 12th, 14th and 17th as he came in with a 68 to close on seven under, a score that looked an unlikely prospect after the Spaniard had opened with a 71 on Thursday.
Mackenzie Hughes, who was tied for the 54-hole lead at Torrey Pines last month before stumbling to a 77 on Sunday, remained in the hunt for the second major running as he birdied the last to post a 68 to make it two Canadians in the top six.
Dylan Frittelli also closed with a three at 18 to rescue a 70 which kept him at seven under, with Cameron Smith (68), Justin Harding (70) and charismatic German Marcel Siem (70) all in a share of ninth place on six under par.