Koepka returned a one-under-par 71 along with joint overnight leader Branden Grace. "Obviously, I have never been in this position before, so tomorrow, I'm just going to go and play and do my best".
South African Branden Grace carded three eagles on his way to a seven-under-par 65 for a share of the lead with American Chase Koepka after the first round of the European Tour's South African Open on Thursday.
Young South African Jacques Kruyswijk is their nearest challenger on nine under par after a 67 and in what is his rookie season on the European Tour.
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 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. I certainly hope that I can keep that trend (of Englishmen winning the SA Open) going". "I just stayed patient after that first hole", he said.
Branden Grace will partner Paisley in Sunday's final round after a six-under 66
He has eight European Tour career victories but has yet to lift his country's Open championship, which is the second oldest in the world having first been formally played in 1903. "It feels good. I don't want to get ahead of myself, just want to do my thing, but this is great for the confidence".
Defending champion Graeme Storm of England, who pipped golf superstar Rory McIlroy of Northern Ireland in a play-off past year to win the second oldest national championship after the British Open, carded a 75.
Koepka began with a bogey and added three more.
South African Retief Goosen, one of the former Major winners, was seven under for the day as he teed off on 18, only to suffer a double-bogey six that left him five strokes off the pace.
South Africa's Darren Fichardt is a shot further back after surging through the field with a brilliant 64 which featured eight birdies in the last 12 holes.
Five-time European Tour victor Darren Fichardt of South Africa is alone in sixth on 205 after a 64, the lowest score of the day, and bettered only by Saddier this week.