AP photo by Peter Morrison / Louis Oosthuizen looks at his putting line on the 17th green at Royal St. George's Golf Club during the third round of the British Open on Saturday in Sandwich, England.

SANDWICH, England — So flawless for so long at this British Open, Louis Oosthuizen started making his worst swings of the week as his two-shot lead evaporated Saturday afternoon.

With an hour left in a third round as undulating as the fairways at Royal St. George's Golf Club, Oosthuizen found himself tied for the lead with Collin Morikawa and Jordan Spieth and facing a 10-foot par putt to avoid falling behind.

"I was a bit upset there," the 38-year-old South African said, "but got myself quickly together."

Oosthuizen delivered a key par save on the 15th hole and an eight-foot birdie on the par-3 16th. He finished with a 1-under-par 69, leaving him with a one-stroke lead and another good shot at ending his 11-year wait for a second major title.

"You know, finishing second isn't great, so I will play my heart out tomorrow and see if I can lift the claret jug again," said Oosthuizen, who won the 2010 British Open on the Old Course at St. Andrews.

Saturday ended pretty much as it had started, with Oosthuizen, Morikawa and Spieth occupying the top three spots and three shots separating them.

Plenty happened in between, however — especially on the back nine and most tellingly on the 18th green, where Spieth missed a par putt from two feet for his second straight bogey to drop three shots back again.

Barely 45 minutes earlier, he was just off the green in two at the par-5 14th hole and seemingly on the brink of taking the lead. After making only par there and then carding bogeys on Nos. 17 and 18, Spieth ran off for some putting practice after a round of 69. The R&A said he declined all media requests; according to Golf Digest, the 27-year-old Texan with three major titles did stop for a few photos with fans on his way out.

It wasn't long before Morikawa and Oosthuizen, each a one-time major winner but some 10 years apart in those triumphs, were walking onto the green on No. 18. Oosthuizen's long putt over a hump from the right of the green settled close to the hole for his par, and then Morikawa slid a 12-foot birdie putt just past the hole.

Morikawa, the PGA Championship winner last year and in his first major test in links golf, settled for a 68 and trimmed Oosthuizen's lead to one shot. He had been four shots behind after 10 holes and made a strong push about the time Oosthuizen showed signs of fading.

"I don't have much experience on links golf, and pretty much all the highlights in my head are from this week," the 24-year-old Los Angeles native said. "Thankfully there is quite a few. Hopefully we can just use that momentum from the first three days and just bring it into the last 18."

Oosthuizen, a runner-up in the past two majors, was at 12-under 198 and on course to be the first wire-to-wire winner at golf's oldest major championship since Rory McIlroy in 2014.

Baylor School graduate Harris English, after an impressive 65 on Friday to make the cut, was tied for 58th after a 72 that put him at 2 over for the tournament.

The pin positions — rather than the weather — proved to be the greatest defense at Royal St. George's on a day when the wind didn't get above 10 mph and a cloudless sky with bright sunshine looked sure to bring more low scoring. However, pins were tucked away, sometimes near slopes, while the firmer fairways brought the deep rough and pot bunkers into play.

"Those are some of hardest pin locations collectively I've ever seen," said No. 2-ranked Jon Rahm, who shot 68 and is five shots off the lead as the 26-year-old Spaniard tries to win a second straight major title after breaking through at last month's U.S. Open.

A number of other big names were unable to launch a challenge. Top-ranked Dustin Johnson started four shots off the lead but plunged out of contention by making five bogeys in his opening 11 holes. Two late birdies netted only a 73, leaving him eight strokes out of the lead. Brooks Koepka, a four-time major champion, was a shot further back after managing a round of only 72.

McIlroy started much further back but reached the turn at 4 under overall after making five birdies. The back nine was another story, and McIlroy threw an iron to the ground — he called it a "little toss" — during a run of three bogeys in five holes on his way to a 69, his first round in the 60s at Royal St. George's. It left him only 1 under and with no chance of a second claret jug.

Instead, Scottie Scheffler (69) and Canada's Corey Conners (66) moved into contention at 8 under while Rahm remained in the fight another stroke back and tied for sixth with Canada's MacKenzie Hughes (68) and South Africa's Dylan Frittelli (70).

All of them are chasing Oosthuizen, who has rung up a career Grand Slam of runner-up finishes since his British Open win. That includes the U.S. Open last month, when he was leading with two holes to play until Rahm's birdie-birdie finish. He also was runner-up in the PGA Championship to Phil Mickelson in May.

He has enough experience with this sort of scenario to know what he intended to do between his 54th and 55th holes.

"I don't really change my routine whether I've got a two-shot lead or I'm trailing by eight, Oosthuizen said. "You know, the only thing that differs is the tee time. I might be up way earlier tomorrow."