Grace's round of seven under par saw him join American Chase Koepka at the top of the leader board and one stroke clear of England's Chris Paisley.
Saddier, a 25-year-old whose only professional success came in the second-tier European Challenge circuit two years ago, scored an eagle and seven birdies to roar into contention.
The double US Open champion went out with three birdies and an eagle on his front nine, and added three birdies and a bogey on the back before arriving on the par-four 18th, where a double bogey made for a frustrating end to his charge as he signed for a 67.
"The finish was awesome after making three birdies in a row on 15, 16 and 17", said Saddier.
"I didn't know, I have a joint course record but it's quite cool".
Englishman Paisley and French counterpart Saddier share the lead on 13 under par after returning respective scores of 65 and 63.
"I can not wait for the weekend", said the Englishman.
Kirk played bogey-free on a gorgeous day along the shores just up the road from Waikiki Beach and had a one-shot lead over Vaughn Taylor, Kyle Stanley and Talor Gooch among the early starters at Waialae Country Club. "I'm really proud with how I've played, I felt really calm and never really felt stressed".
"At the moment it is weird - I feel like I did not play awesome golf".
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Compatriot Graeme Storm won the tournament past year after a play-off with superstar Rory McIlroy of Northern Ireland, but failed by one shot to make the halfway cut Friday.
"I am very happy, I just wish I had more holes to play because I feel I could have birdies every hole today", said the 2017 Joburg Open champion, who was also frustrated with a slow start.
He eagled the second, then birdied three of four holes from the fifth and four of five from the 12th.
Retief Goosen, a two-time US Open victor, faltered at the last on Friday with a double-bogey six and lost further ground on the field with a one-over 73 to finish at seven under heading into the final round, while Ernie Els carded a two-under 70 to sit 10 shots off the lead at five under.
And Jamie Donaldson recorded the first hole-in-one of this year's tournament when he aced the par-three 14th with a nine iron.
Paisley, who shared a four-shot overnight lead with Adrien Saddier of France, carded a two-under 70 over the 6,942-metre/7.594-yard layout 12 kilometres (7.5 miles) east of Johannesburg.
South Africa's Jacques Kruyswijk holds solo third place on 9 under.
But ex-US Masters champions Mike Weir (146) of Canada and Trevor Immelman (149) of South Africa bowed out after the second round.