UAE emerges as a hub for innovation
The Australian skipper became the second fastest batsman to reach 6,000 Test runs in his 111th innings, second only to the legendary Don Bradman.
Smith, with three centuries in this five-match series, has been the bane of the England team and was potentially heading towards another ton.
At the close, Smith was on 44 with Khawaja reaching his highest score of the series on 91 in Australia's 193 for two in reply to England's 346 first innings.
"We're stoked he's on our team so we don't have to bowl at him," Australia paceman Pat Cummins said of Smith.
"Those big innings he has played have been really important. In Brisbane and Perth they were match-winning and in Melbourne it was match-saving.
"It's just incredible and I think he's been the difference between the two sides."
So far in the series Smith has scored 648 runs at 162.
The pair shared in a 107-run partnership after the loss of openers Cameron Bancroft and David Warner.
Khawaja raised his third fifty of the series with a six down the ground off Moeen Ali.
"Great reception when he walked back in, all just really happy for him," Cummins said of Khawaja's knock.
"We know how well he's been hitting them all summer, he just hasn't got that big score to get him away."
Debutant Mason Crane, the youngest specialist spinner at 20 to play for England in 90 years, troubled Smith and Khawaja with his wrist spin, enticing some edges and false shots to finish with a creditable no wicket for 58 off 17 overs.
"It was awesome. Naturally I was a little bit nervous, but I was also really excited," Crane said. "I had great fun out here and I can't wait until tomorrow.
"There were periods where we had a couple of inside edges that didn't quite get to short leg or to slip, but that happens and you keep plugging away and hopefully they go my way tomorrow."
Warner reached his fourth half-century of the series before James Anderson coaxed an edge behind for 56 after Bancroft was bowled by Stuart Broad for a seven-ball duck.
England's maligned tail earlier piled on runs before the tourists were bowled out at lunch.
Newcomer Tom Curran smacked 39 off 65 balls, Moeen Ali hit 30 and Broad clubbed two sixes in his 31 off 32 balls as England lashed out late in the extended morning session.
Smith pulled off a stunning one-handed catch to dismiss Dawid Malan for 62, but the Australians were guilty of two embarrassing dropped catches to give England a helping hand.
But Australia then dropped two regulation catches in the space of four balls.
Cummins put down Curran on 21 at mid-on off Nathan Lyon then in Cummins' next over Josh Hazlewood made a dreadful hash of a skied catch at mid-on from Moeen on 22.
It was the 11th dropped catch by Australia's fielders in the series, with five coming off Lyon's bowling.
But Cummins made partial amends when he had Moeen caught off his glove to a lifter for 30, giving wicketkeeper Tim Paine his fourth catch of the innings.
Broad and Curran hit out late in the session with Broad clubbing Cummins for two sixes to the delight of England's Barmy Army supporters.
Curran finally fell to a bat-pad catch by Bancroft for 39 off 65 balls to give Cummins his fourth wicket.
Broad went for another heave on 31, only to top-edge behind the wicket for Smith to take the catch and give spinner Lyon his only wicket.
Australia have already clinched the Ashes with an unassailable 3-0 lead in the five-Test series.
England first innings (overnight 233-5)
D. Malan c Smith b Starc 62
M. Ali c Paine b Cummins 30
T. Curran c Bancroft b Cummins 39
S. Broad c Smith b Lyon 31
M. Crane run out 4
J. Anderson not out 0
Extras: (lb2, w2) 4
Total: (all out; 112.3 overs) 346
Fall: 1-28, 2-88, 3-95, 4-228, 5-233, 6-251, 7-294, 8-335, 9-346, 10-346.
Bowling: Starc 21-6-80-2, Hazlewood 23-4-65-2, Cummins 24.3-5-80-4 (2w), Lyon 37-5-86-1, M. Marsh 7-0-33-0
Australia first innings
C. Bancroft b Broad 0
D. Warner c Bairstow b Anderson 56
U. Khawaja not out 91
S. Smith not out 44
Extras: (b1, nb1) 2
Total: (2 wickets; 67 overs) 193
Fall: 1-1, 2-86.
Bowling: Anderson 14-4-25-1, Broad 10-2-28-1, Ali 17-3-51-0, Curran 8-2-26-0, Crane 17-0-58-0 (1nb), Root 1-0-4-0
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