CENTURION - As hunches go, picking Shaun Marsh for this tour was a biggie. He had averaged 25 in first-class cricket since being axed from the Test team in early 2012. He was the Sheffield Shield's 34th highest run scorer this season. But whatever Australia's selectors saw in Marsh that no one else did, whatever encouraged them to pencil him in at No.4 against the world's best team, whatever inkling they had, it paid off on the first day of the series in South Africa.By the close of play, Marsh and Steven Smith had demoralised the South Africans and in particular Graeme Smith, who had sent Australia in after vacillating over the decision in the lead-up to the toss. His bowlers troubled Australia in the opening session and took three wickets before lunch but their threat level diminished as the ball grew older and they managed only one wicket for the rest of the day, far from a passable outcome.From 98 for 4 when Michael Clarke top-edged a hook to fine leg off Dale Steyn, the Australians had moved on to ??? at stumps, with Marsh unbeaten on 122 and Smith on 91, eyeing off a possible third Test century from four matches. Already the top order had bettered their first-innings efforts in all five home Ashes Tests, for 257 was their highest score at five down in those games. Their frequent saviour Brad Haddin will not know what to do with himself in Centurion.The Marsh-Smith partnership was 199 at stumps and the runs had flowed more and more rapidly as the day wore on. The second new ball only served to bring more boundaries for the two well-set batsmen, and the frustration showed on the faces of the South Africans as the shadows grew longer, not that they had created vast numbers of chances.Marsh's only genuine life came on 12 when he cut Morne Morkel hard and Hashim Amla at gully put down a difficult chance. There were other nervy moments, including a miscue just over the head of a jumping Robin Peterson at cover on 57, a hook that narrowly eluded a diving Morkel at fine leg on 77, and a run-out chance on 83, but Marsh survived.He not only survived, he thrived. Strong when driving down the ground and through the off side, he worked plenty of singles through leg when the bowling was too straight. Most of his strokes were along the ground and he brought up his second Test hundred from his 174th delivery with a tickle to fine leg off the part-time offspin of JP Duminy, celebrating like a man who had been given a second chance at life, which in cricketing terms he was.However valid the arguments were against Marsh's inclusion, they had become irrelevant. And if Shane Watson returns from his calf injury for the second Test, it will not be Marsh who makes way. At least, not unless his own calf problem, which appeared to hamper his running at times during his century, acts up. Not that he was the only man Australia had to thank for their solid scoreline at the close of play. Smith showed again why he now tops the list of Australia's Test batting averages over the past year.As he did in his mature Ashes hundred at the WACA, Smith left the ball with fine judgment and waited for the bad balls to score from. He drove through cover with power when the bowlers overpitched and was also strong down the ground, and brought up his half-century from his 118th delivery with a punchy cover-drive for four off Ryan McLaren. Earlier, McLaren had bowled well on the greenish-tinged pitch but like his team-mates, the threat was blunted after lunch.Graeme Smith must have gone to that first break pleased with his decision to send the Australians in, for his men had them at 83 for 3. His bowlers had just enough movement and, in the case of Morkel, disconcerting bounce. An over-confident David Warner, who had struck two boundaries in the first over of the game, played on for 12 when he tried to cut a Steyn ball that was both too straight and too full, and Australia were 15 for 1.That became 24 for 2 when Morkel came on. His first ball was a bouncer that struck Chris Rogers on the arm and the follow-up delivery was another short one that Rogers, on 4, could only fend to short leg for Duminy to take a sharp catch. The composure shown by Alex Doolan in his first Test was encouraging for the Australians; he punched a cover drive for four off Vernon Philander and cut another hard off Steyn.Generally, Doolan looked up to the task of Test cricket, but converting his classy starts has always been his issue at domestic level and on 27 he pulled McLaren straight to Alviro Petersen at midwicket. Clarke, who shifted himself down to No.5 to accommodate Doolan and Marsh, reached 23 before he fell to a clear bouncer plan from Steyn, his top edge finding Philander running around at fine leg.At 98 for 4 shortly after lunch, it looked like smooth sailing for South Africa. But Steyn, battling a stomach bug, picked up no more wickets, Philander battled to find the swing and seam he had with the new ball, Morkel was handled more easily by Marsh and Smith, McLaren's threat diminished and Peterson was treated like a part-timer, literally, for he and Duminy each finished the day with 0 for 34 from 10 overs.Graeme Smith must have walked off at stumps regretting the advantage he had given Australia with his decision at the toss. It will be up to him and his fellow batsmen over the next few days to ensure it does not cost South Africa the match.