Six years after his Supercars debut and in his 165th race start, Pye defied testing conditions and a relentless Jamie Whincup to savour victory in Saturday's 25-lap thriller in Melbourne.

Pye fought back tears and dedicated the long-awaited win to his late dad after what he described as "the drive of my life".

He held on to claim line honours 0.26 of a second ahead of seven-time series champion Whincup in race three of the Australian Grand Prix support event.

Holden's Nick Percat was third in a race that ended in darkness after showers lashed the track.

It was also the first win for Pye's Walkinshaw Andretti United team in their debut season.

The once mighty Walkinshaw Racing linked with US motorsport legend Michael Andretti and British operation United Autosports which is part-owned by McLaren Formula One chief Zak Brown this season after a horror 12 months.

Walkinshaw Racing lost Holden factory backing for the first time in 26 years and went through last season without a win.

"It has to be (best drive of life). It means the world. It felt like it's taken a long time," an emotional Pye said.

"My dad's up there (in heaven) he could have helped a bit more with the weather, but yeah, obviously I miss him and this is for him today.

"It's my first win. I am just speechless.

"That was a tough race and to have someone like Jamie behind you is a mega win."

McLaren team chief Brown was trackside ahead of Sunday's Australian Grand Prix and could not hide his excitement.

"I'm happy to be here. I'm going to come more often," he said.

"I think it's as good a drive as I've ever seen - he earned it."

Pole sitter Scott McLaughlin of Ford's DJR Team Penske finished 15th after locking up and overshooting turn one on the second lap and failing to recover when he opted to stop for wet tyres.

Whincup earlier earned his first Supercars win of the season in Saturday's opening race at Albert Park.

His mastery in the rain allowed him to lead from pole position until the chequered flag and was never seriously threatened by McLaughlin or DJR Team Penske teammate Fabian Coulthard who completed the podium.

Australian Associated Press