Koepka set the mark in the morning wave with a round that included seven birdies and an eagle against two dropped shots while Grace’s rollercoaster score came in the afternoon.
The South African added three birdies and two bogeys to eagles at the par-5 second, eighth and 15th for a 65 that sees he and Koepka one ahead of England’s Chris Paisley.
A group of four players sits two shots off the lead on a day which saw 79 players break par.
Grace said he couldn’t recall ever making three eagles in a single round but was pleased to have posted a low number as he tries to capture the title for the first time.
“I cant remember the last time – or if I’ve ever made three eagles in a round,” he said with a laugh.
“But it was fun. Obviously a little bit rusty with some of the wedge shots and things like that but all in all very pleased with the outcome and I think I pretty much got the most out of that round.
“All the par 5s I pretty much hit great tee shots and really gave myself some chances. The only one I really was aggressive with was the second hole.
“I got aggressive with the 3-wood and knocked it in there 25 feet and managed to roll in the putt.”
American Chase Koepka shares the opening round lead in South Africa.
He said that putt had given him confidence for the rest of the round despite struggling with his speed control.
“I get on a streak like this where if I make one of those longish putts in the first couple of holes pretty much then I just have to get it there,” he said.
“I left a few short today. I had them all on line but the speed was just a little bit short on some of them.”
For Koepka, who is following his brother’s footsteps by opting to learn his craft in Europe and graduating from the Challenge Tour last year, good short iron play was the key.
"Inside 150 yards, I was really good today,” he said. “I didn't have too many shots from inside 150, but every one that I had, I was hitting them close.
“The putter was really hot on the front nine and then I had a bunch of short putts on the back nine which helped a little bit.”
It was a disappointing start for the two Australians in the field, Brett Rumford signing for a 1-under 71 to be in a share of 59th while Jason Scrivener did well to salvage a 74 after dropping five shots in his first six holes.
Scrivener, who was born in in South Africa before moving to Australia at the age of five, dropped shots at the first, second and fifth then made a double bogey at the sixth.
A birdie at the 8th and a run of three straight from the 13th had him back on track but a late disappointment awaited with a final bogey of the day at the 16th.
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