Pace spearhead Ryan Harris will be given until today to prove he can carry an ongoing knee injury through the most important match of his career.
The 34-year-old Queensland quick battled soreness following Australia's victory in the second Ashes Test on Monday in Adelaide.
Mitchell Johnson has won consecutive man-of-the-match awards to skittle England's brittle batting, but the importance of Harris is massive.
Harris has claimed 32 wickets at 19.75 in 2013, second among Australians to pace partner Peter Siddle on 36 victims. Siddle has played 12 Tests to Harris' six.
Australia are yet to name a side for today's potential Ashes-clinching Test, with pace duo Doug Bollinger and uncapped Nathan Coulter-Nile on standby.
Pace-bowling all-rounder James Faulkner was also pushing for a possible recall to the side before suffering a fracture of his right thumb while batting in the nets yesterday.
Off-spinner Nathan Lyon is under pressure to retain his place in the side on a WACA pitch expected to be fast and bouncy with lots of carry for the quicks.
Australia are pushing for a third consecutive win which would allow Michael Clarke's men to reclaim the Ashes in what would be a massive celebration for the 32-year-old skipper in his 100th Test.
However, the fitness doubt over Harris and Faulkner's injury have muddied the waters for Australia.
"I'm going to wait until the toss and see how everybody pulls up after training and it gives us another opportunity to have a look at the wicket," Clarke said yesterday.
"Ryan bowled today so we're going to have to wait and see how he pulls up.
"I'm confident he'll be fine."
Clarke's captaincy has polarised fans and there have been plenty of ups and downs, including series defeats in India and England in 2013.
The skipper says the team's success remains his key focus.
"That's why it's a most special match for me," he said.
"It's the fact that we have a chance to win the Ashes."
England have reached 200 once in four innings in the series but Clarke says inserting the struggling tourists in is unlikely.
"Unless the wicket is extremely green, personally I find it hard to send a team in," Clarke said.
"It looks like a pretty good wicket to me."
Clarke paid tribute to his opposite number Alastair Cook, who's also reaching the 100-Test milestone this afternoon.
"He deserves a lot of credit for the success he's had over a long period of time for England," Clarke said.
Cook has misfired with the bat on this tour and has lost consecutive matches as captain to Clarke's side.
England have beaten Australia in Perth once in 12 attempts and that was in 1978 against a home side depleted by World Series Cricket.
A top of 38 degrees is forecast today and Cook is definitely feeling the heat, admitting to sleepless nights this week with the tough role "when things don't go well". AAP