South Africa's Louis Oosthuizen remained the man to catch midway through the third round of the 149th Open at Royal St George's.

Oosthuizen held a two-shot overnight lead thanks to a championship record halfway total of 129 and shrugged off a slow start to maintain that cushion ahead of three-time major winner Jordan Spieth.

Spieth, whose last major title came in the 2017 Open at Royal Birkdale, birdied the second and fourth and recovered from a dropped shot on the fifth with further gains on the sixth and seventh to move into a share of the lead.

That proved short-lived however as Oosthuizen ended a run of six straight pars by also two-putting the par-five seventh for birdie and another gain on the ninth restored his overnight advantage on 13 under.

Compatriot Dylan Frittelli and former US PGA champion Collin Morikawa were four shots off the pace, the latter recovering from two early bogeys to birdie the seventh and eighth.

Former Open winner Rory McIlroy had earlier thrown a club in frustration after squandering an ideal start to his third round.

McIlroy, who made the halfway cut with just a shot to spare after consecutive rounds of 70, climbed into the top 20 after firing five birdies in a front nine of 31, only to struggle home in 38 for a disappointing 69.

The 32-year-old, who is without a major title since 2014, dropped shots on the 11th and 13th and then hurled his long iron to the ground after pulling his tee shot on the par-five 14th into the rough.

McIlroy could only smile wryly when his par putt on the next horseshoed around the hole and stayed out, but the former world number one will not be happy with letting a promising position slip through his fingers.

"The front nine was great but it's turning that nine holes of golf into 18 holes and then turning 18 holes into an entire tournament," McIlroy said. "There are signs there it is going in the right direction, unfortunately I couldn't keep it going for the rest of the round."

Scotland's Robert MacIntyre had completed a 65 in brilliant fashion, picking up shots on the 14th and 17th before holing from 60 feet for another birdie on the 18th from a similar location to where Sandy Lyle almost came to grief on the 72nd hole of his title win in 1985.

"Obviously it was a bonus there at 18," said the 24-year-old left-hander, who was sixth on his Open debut at Royal Portrush in 2019 and has made the cut in all seven of his major appearances to date.

"You are not expecting to hole that, you are just trying to get down in two. But my pace was great all day and I managed to put good pace on it, had a decent line and it went in."

Bryson DeChambeau, whose complaint following his opening round that his driver "sucks" prompted an angry response from his club manufacturers, was in the second group out on Saturday and shot 72 to finish three over.