American rookie Ryggs Johnston booked a shootout with home hero Lucas Herbert for the Australian Open trophy at waterlogged Kingston Heath on Saturday as favourite Cameron Smith crashed out of contention.
In the women's component of the dual gender event, South Korean former world No 1 Shin Jiyai heads into Sunday with a two-stroke lead over Australia's Hannah Green.
Johnston, named after Martin Riggs, the fictional lead played by Mel Gibson in the Hollywood cop movie "Lethal Weapon", had guns blazing in a four-under 68 to finish the day level with Herbert on 14-under for a total of 201.
The pair held a two-stroke lead at the sandbelt course in Melbourne from a group on 12-under, including Chilean defending champion Joaquin Niemann and home hope Elvis Smylie, who won the Australian PGA Championship in Brisbane on Sunday.
Johnston, who hunts elk and deer in his home state of Montana, will target a first win as a professional on Sunday.
"It’s been a long time since I won a golf tournament," said the 24-year-old.
"It would be huge for me to win, for my career. But I'm trying to keep as much pressure off myself as I can."
Despite missing out on a practice round at Kingston Heath because of a storm on Wednesday, Johnston shot a seven-under 65 in the first round and on Saturday stormed into a two-shot lead over Herbert after racking up six birdies in his first 12 holes.
But a poor tee-shot led to a double-bogey on the par-five 14th, allowing Herbert a share of the lead."
Overnight leader Herbert had started the day with a four-stroke cushion but had a vexing day on the greens in an even-par 72 as rain poured through the afternoon.
"Frustrating day, but I haven't gone backwards," said Herbert, whose one birdie on the par-four ninth cancelled out a lone bogey on the fourth.
"I'm still massively in it tomorrow .... I've just got to stay patient, it's a tournament you really want to win as an Aussie. So it can be easy to want it too much, try too hard."
British Open winner Smith's long win drought is almost certain to continue after he stumbled through a horror 76 laden with five bogeys, leaving him 12 strokes adrift of the leaders.
The former world No 2 came home to Australia looking for a first win in nearly 16 months but has missed out in four tournaments.
Bulky Australian Marc Leishman, a six-time winner on the US PGA Tour who now plays on the LIV Golf circuit, moved into contention with a four-under 68 to be three shots off the pace.
No Australian woman has won their national Open since Karrie Webb in 2014, and Shin is well-placed to prolong the drought.
She shot a six-under 67 to be 14-under (204) for the tournament, making birdies on four of the last six holes.
Major winner Green had held the lead coming into the last stretch but crashed with a double-bogey on the 17th in her round of 68, allowing Shin to pick up three strokes with a birdie on the same hole.
South Africa's double defending champion Ashleigh Buhai remains in contention, four off the pace, after firing a 67.