Australia's Steve Smith ruled out of third Ashes Test

Smith, who captained the team to emphatic eight-wicket wins in Perth and Brisbane, missed training on Monday citing illness but batted on Tuesday and was initially named in the starting XI.

He was on the field at Adelaide Oval but walked off 20 minutes before the toss shaking his head after a chat with coach Andrew McDonald.

"Over the past few days he has been feeling unwell, with symptoms including nausea and dizziness," Cricket Australia said.

"He was assessed and monitored closely and was close to being available to play.

"However, given the persistence of symptoms, a decision was made not to proceed."

The governing body said Smith was being treated for "a potential vestibular issue", which is essentially vertigo, a problem he has suffered from before.

"This is something Steve has experienced intermittently in the past and is being managed accordingly," Cricket Australia added.

"He is expected to be available for the Boxing Day Test in Melbourne."

Smith has long been Australia's middle-order rock, a player with 36 Test centuries and more than 10,000 runs and his absence is a huge blow.

It handed a lifeline to 85-Test veteran Usman Khawaja, who had been axed as opener with selectors opting to stick with Travis Head and Jake Weatherald.

Khawaja came in for Smith at four and was 41 not out at lunch with Australia reaching 94-2.

"Pretty lucky we've got someone like Uzzie (Khawaja) who can step right in," said skipper Pat Cummins, who won the toss and chose to bat.

Smith's absence capped a dramatic build-up to the third Test in which Australia is chasing an unassailable 3-0 series lead and England battling to keep the Ashes alive.