Paul Dunne is seven strokes adrift of early leader Paul Peterson in the weather-delayed Joburg Open at Royal Johannesburg and Kensington Golf Club.
The American fired a sensational nine-under-par 62 to take a two-shot lead before play was suspended due to a waterlogged course.
Peterson took advantage of the early dry conditions, moving to nine under on the West Course before the heavens opened again and play was suspended at 2.35pm with 98 players still out on the course.
With the rain continuing to fall heavily, play was abandoned for the day within an hour and was scheduled to resume at 6.45am on Friday morning.
Dunne was tied 46th on two-under-par after 15 holes on the East Course, mixing birdies at the fifth and sixth with 13 pars.
Royal Dublin's Jeff Hopkins, who came through the Joburg Open qualifier on Wednesday, was tied 78th on one-under-par through six holes after following an opening birdie on the West Course with five pars.
Former Ryder Cup captain and Open champion Darren Clarke was back in joint 162nd on two over with four holes to play on the East Course.
The Dungannon veteran, 48, bogeyed the second but then birdied the seventh and eighth to turn in one-under-par 36.
But he bogeyed the 10th and double bogeyed the 11th to slip to two-over-par before play was suspended for the day.
Peterson had matching nines of 31 to sit two shots ahead of English pair Aaron Rai and Paul Waring, who both played the East Course that will be used for the full field after the cut.
Both will be seeking a spot at The Open Championship that's on offer for the top three players in the top ten at the end of the week who are not already exempt for Royal Birkdale in July.
"It was fun out there today," 28-year-old Peterson said. "Birdieing half of the holes is never bad and bogey-free to match, so I felt really good out there.
"I got into a nice rhythm, and I put a new putter in play this week. It proved to be a good first round with it, so I'm really happy.
"Thankfully we were in the fairway a lot, ball in hand always nice but I feel like I've been playing really well the last few weeks and I feel like a round like this has been building, so to have it happen sooner than I thought is great.
"I'm feeling like I'm in a good place right now."
Swede Sebastian Soderberg was then at six under with a single bogey in his 66 on the East, while Frenchman Romain Langasque fired a 65 on the par 71 West, with an eagle on the ninth.
Jbe Kruger is one of just three players in the field already exempt for the year's third Major Championship at Royal Birkdale, and he too fired a 65 on the West with birdies on the second, third, sixth, ninth, 14th and 15th.
Home trio Dean Burmester, Dylan Frittelli and Teaghan Gauche, Italian Francesco Laporta and Spaniard Pep Angles were then all at five under alongside George Coetzee and Adilson da Silva, who had five and six holes remaining respectively.
English trio Paul Maddy, Chris Paisley and Jordan Smith, Frenchmen Damien Perrier and Joël Stalter, Scot Marc Warren and South African Jaco Prinsloo were in the clubhouse five shots off the lead, with defending champion Haydn Porteous in that group having completed 13 holes.