With the two major parties running neck-and-neck, Independent member for Kalgoorlie John Bowler has emerged to hold the whip hand if a hung parliament materialises.
Both Labor and the Liberals are set to fall just short of an absolute majority, giving the former Labor MP a key role.
Mr Bowler, who was sacked from cabinet over his association with lobbyist Julian Grill, said he would vote with whatever party delivered for his electorate.
"I never thought I'd say this... but I could vote with Colin Barnett," he said.
There has been a swing to the Liberals, with the party set to pick up a swag of marginal seats.
Despite the closeness of the results, the ABC called the election for the Liberals at 9.35pm.
However, the biggest surprise for much of the evening looked to be in the safe Labor seat of Fremantle, home to Attorney General and Health Minister Jim McGinty.
With about 70pc of the vote counted last night, he had 39pc of the primary vote.
But Greens candidate Adele Carles is in a tight race with the Liberals Brian Christie for second spot.
If Ms Carles were to overtake Mr Christie, the latter's preferences could hand her the seat.
In Kingsley, the state's most marginal seat, Liberal candidate Andrea Mitchell will take the seat with more than three-quarters of the primary votes counted.
She needed a swing of just 0.1pc to topple sitting Labor member Judy Hughes.
Ms Hughes told 6PR Radio that a loss was "definitely on the cards".
"It was always going to be a 50-50 chance," she said.
The Liberals' Tony Simpson looks all but home in the notionally Labor seat of Darling Range, while John Castrilli, the Liberal member for Bunbury, will take the seat.
Bunbury was notionally Labor after the electoral redistribution following the one vote-one value reforms.
In the battle of two sitting MPs in Collie-Preston, Labor's Mick Murray has a narrow lead over the Liberals' Steve Thomas, but preferences will decide the result.
Mr Murray is not claiming victory, saying "there's a long way to go".
Preferences will also decide Riverton, where the one of the Liberals' star recruits Mike Nahan has a narrow lead over former minister Tony McRae. The Green vote is running at more than 10pc.
Mr McRae said voters wanted to know where the benefits of the boom had gone.
The Liberals' Albert Jacob has almost 50pc of the primary vote in his battle with Labor's Louise Durack in Ocean Reef.
Frank Alban, the Liberal candidate for Swan Hills, looks to have done enough to take Swan Hills from Labor rival Graham Giffard, but the Greens vote is running at almost 20pc.
Further south, the Liberals' Joe Francis is clearly ahead of Labor's Anne Wood in Jandakot, but with a strong Green vote also the outcome could be a lot closer after the preference distribution.
Up in North-West, Upper House Labor MP Vince Catania is locked in a tight three-way battle with the Liberals' Rod Sweetman and the Nationals' Tom Day.
Mr Sweetman looks favoured on preferences, despite running 10pc behind in second.
Sitting Geraldton MP, Labor's Shane Hill, is in trouble in what is now a notionally Liberal seat, with Ian Blayney poised to win.
In Morley, Premier Alan Capenter's hand-picked candidate Reece Whitby seems likely to need preferences from former Labor MP John D'Orazio, who has preferenced the Liberal candidate Ian Britza.
In nearby Mount Lawley, another hand-picked candidate Karen Brown is virtually certain to lose to Liberal candidate, and Perth deputy mayor, Michael Sutherland.
The Liberals' Andrew Partington could see Nationals preferences help him defeat Peter Watson in Albany.
A fascinating struggle is developing in Kwinana, where Labor's Roger Cook could have a tough battle with Independent, and Kwinana mayor, Carol Adams, who was passed over for the Labor pre-selection.
Ms Adams said she was confident of picking up Liberal and Greens preferences.