Australia opener Travis Head hit back at a majestic 160 from England's Joe Root on the second day of the fifth Ashes Test in Sydney.

Root's masterclass helped England to a competitive 384 at the Sydney Cricket Ground, but Head feasted on some woeful bowling to skip to an unbeaten 91 from 87 balls. Australia are 166-2 after 34.1 overs and have cut the deficit to 218.

The tourists had earlier lost 7-158, including a final collapse of 5-61 sparked by Jamie Smith's horrific dismissal.

Smith, who was caught off a no-ball and had other moments of fortune in his skittish 46, played a truly awful shot to be caught off part-time medium-pacer Marnus Labuschagne.

On a pitch that offered more for the bowlers on Monday, England's total felt around par, and Australia's batters made rapid inroads, helped by the tourists' attack and two dropped catches.

With new-ball bowlers Brydon Carse and Matthew Potts leaking runs, Jake Weatherald was put down twice as he and Head added 50 in 9.2 overs.

It took the belated introduction of captain Ben Stokes to trap Weatherald lbw, with Head then sharing another 105 with Labuschagne. Stokes and Labuschagne exchanged words before the England captain drew him into an edge to gully for 48.

Head remains, on course for his third century of the series. Of equal concern for England would have been the fitness of Root, but a spell off the field at the end of the day was later explained as cramp in his back.

Hyped and anticipated, this has turned into a poor Ashes series, low on quality, individual brilliance and magic moments.

The second day of this finale was in keeping with what came before. Root and Head elevated themselves above the rest and Michael Neser took 4-60 in an improved Australia bowling performance, but England were slapdash with the ball and Smith's dismissal was atrocious.

Even the obvious Australian plan was bizarre. Labuschagne, who had not before taken a Test wicket with his seam, had seven fielders on the boundary for some gentle bouncers.

Still, Smith continued his poor tour with a forehand to Scott Boland at deepish cover. It was a dismissal that encapsulated the carelessness that so often costs this England team.

Only three times has a team made more than England's 384 batting first in a Test on this ground and lost, and the pitch is already showing signs of uneven bounce.

But Australia have the prolific Head, the opportunity to go beyond England's score on Tuesday and the potential of pressuring the fragile visitors in the third innings.

Head's elevation to open in the second innings of the first Test in Perth was the defining moment of the series and the left-hander is tucking into England's bowling once more.

Somehow, England continue to feed his strength outside off stump. Arguably neither Carse nor Potts - the latter playing his first Test for more than a year - should have been opening the bowling, and they let Australia get off to a flyer.

In fairness, each bowler had Weatherald dropped. Root parried a tough high edge at first slip off Potts when Weatherald had nine, then Ben Duckett shelled a more straightforward chance at mid-wicket off Carse when he had 14.

Stokes, on in the 11th over, ensured neither miss was too costly. With his eighth delivery, the captain had Weatherald play all around a full one.

Following his intervention with the ball, Labuschagne eased into Head's slipstream. Head carved through the off side, Labuschagne played clips and drives. The clouds gathered, Potts' economy was more than eight an over.

As the umpires discussed the light, Stokes and Labuschagne went at each other. Play continued, Stokes got his man, and Head slowed up to leave an overnight wait for his hundred.

Root has been feast or famine in this series. Until this Test, his landmark century in Brisbane was one of only two occasions when he passed 20. In Sydney, the city where he was dropped for the only time in his Test career and once batted himself into hospital in extreme heat, the former captain was outstanding.

Without Root, England would have squandered their promising overnight position of 211-3. Harry Brook played a limp poke at Scott Boland to be caught at slip for 84 and Stokes edged a beauty from Mitchell Starc for an 11-ball duck, meaning the tourists lost 2-3 in four overs.

On one hand, Smith's role in a stand of 94 with Root was valuable, but the wicketkeeper was fortunate to last so long. He was caught off a Cameron Green no-ball on 22, then edged and miscued the same bowler. The dismissal to Labuschagne's bouncer plan as the second new ball approached was an appalling piece of cricket.

By this point, Root had moved to three figures from his overnight 72. After edging Neser over the slips on 94, he drove the same bowler down the ground to join Australia great Ricky Ponting on 41 Test tons - only India's Sachin Tendulkar and South Africa's Jacques Kallis have more. He celebrated with the same shrug of the shoulders he pulled in Brisbane.

On he went, adding 52 for the seventh wicket with Will Jacks. He passed 150 for the 17th time in Tests - only four other players have done so more.

Root eventually offered a leading edge to a diving Neser in the bowler's follow-through, part of England's final collapse of 4-9. After carrying England's batting, perhaps it is no surprise he later suffered discomfort in his back.