Usman Khawaja has gone from his career appearing over to helping stabilise Australia's innings to 2-94 on a dramatic first morning of the third Ashes Test.

Left out of Australia's team on Tuesday, just two days shy of his 39th birthday, Khawaja was granted a lifeline on Wednesday when Steve Smith was ruled out with vertigo.

Batting at No.4, Khawaja was then dropped on five by Harry Brook at second slip, when he drove at a wide ball from recalled English seamer Josh Tongue.

But the left-hander then took control of Australia's innings in the opening session, working his way to an unbeaten 41 at lunch on the opening day.

Khawaja missed the Brisbane Test with a back injury before Australia opted to stick with Travis Head and Jake Weatherald as openers.

With only two scores above 50 in his past 27 Test innings, Khawaja's chances of featuring in Test cricket again had at that point appeared bleak.

But he has looked his old laconic self on Wednesday, clipping Tongue for one boundary through the legside and cutting him for another.

He twice pulled Stokes to the boundary, while two sweeps off spinner Will Jacks also went to the rope.

Marnus Labuschagne was 19 not out at the break.

Khawaja had come to the wicket with Australia in some early trouble at 2-33, after Weatherald and Head fell in consecutive overs in the first hour.

Brydon Carse bowled poorly and was far too wide to Weatherald, who punished him with several cut shots through the offside.

But the left-hander was caught behind when he attempted to pull Jofra Archer and was cramped for room, skying the ball to wicketkeeper Jamie Smith.

Head then followed shortly after, when Zak Crawley did superbly to hang on with his left hand close to the ground at cover, after Head had driven a Carse half-volley.

The hosts can wrap up the series with a win in Adelaide, with victory in four days enough to make for the fastest retention of the Ashes since 1921.