It wasn't Mitch Marsh's highest score, nor was it necessarily his most fluent knock.

But his 298-ball 162 against Pakistan A was the longest innings of the newly-minted joint vice-captain's first-class career and that may be of greater importance to Australia’s depleted Test side.

Marsh went big after posting a half-century the previous day, watchfully crafting his marathon innings after resuming his stand with brother Shaun to finish with a 207-run third-wicket partnership that lasted close to five hours.

The 26-year-old was imperious against a spirited Pakistan A pace attack, driving through the off-side with flair as he struck 17 boundaries, and took his medicine against the two part-time off-break bowlers that have sent down 98 overs between them so far.

A far greater challenge lies ahead against Yasir Shah come the first Test but the sheer determination of Marsh in sweltering heat was cause for encouragement. He now has six centuries and a first-class batting average above 50 since the start of last summer.

The right-hander's previous longest innings, in terms of balls faced, was a 294-ball double-century against India A in the winter of 2014 that helped convince selectors to pick him for the Test side to play Pakistan in Dubai later that year.

Marsh hit 10 sixes in that innings. Of the nearly 300 balls he soaked up against Pakistan A over the course of 104 overs on days two and three here in Dubai, not one of them was a six.

Just about the only ball he hit in the air was his last, drilling a catch straight to cover.

A typically aggressive batsman, Marsh admits it took some restraint to play the way he did.

"The conditions dictate that, it's a pretty hard wicket to score in front of the wicket on so you just had to be really patient," he explained after Australia went to stumps at 4-494 having lost just two wickets all day.

"My game plan today was to be really patient and play every ball on its merits and if it took me the whole day to get my hundred, so be it.

"It was really nice to spend that time in the middle."

Finding the right way to utilise his natural power on a sluggish surface was a challenge expertly met by Marsh, who has worked hard on not pushing as hard at the ball.

"I don't think I flicked a switch at any point, I just continued to work on my game and continued to improve," he continued.

"There's certain elements of my game that have got better and it's nice to put them in place in the middle.

"I've worked hard on having softer hands when playing spin in these conditions … playing the ball later and being really strong in my defence."

After his lengthy partnership with his brother came to an end when Shaun was bowled for 94, Mitch Marsh had a front-row seat to newcomer Travis Head.

The Australians' rhythm was hardly disrupted as left-hander replaced left-hander, with Head taking a further step towards a maiden Test cap by reaching 90 not out at the close.

"He's in really good form and in really good touch, he's proven over the last couple of years he can score big runs for South Australia," said Marsh.

"He's ready to go. He's a great guy around the group. If he gets the opportunity, it'll be a really exciting for him."