Brisbane: Michael Clarke produced his third double-century this year and reached 1,000 runs for 2012 with a defiant unbeaten 218 on Monday, continuing his exemplary batting form since becoming Australia captain to guide the hosts to a first-innings lead against top-ranked South Africa.Clarke posted his 20th career Test century and his sixth since replacing Ricky Ponting as skipper last year, leading Australia to 487 for 4 by stumps on day four in reply to South Africa's 450.After going to the crease with Australia in trouble at 40 for 3 late Sunday, Clarke shared a record 259-run, fourth-wicket partnership with opener Ed Cowan â who posted his maiden Test hundred before he was run out for 136 â and an unbeaten 188-run fifth-wicket stand with Mike Hussey, who was not out 86.A South African attack spearheaded by Dale Steyn (1 for 109) and Vernon Philander (0 for 82) â the top two ranked bowlers in the world â didn't take a wicket in 95 overs.After being in such a precarious situation on day three, the Australians are now chasing an unlikely victory. They'll need to establish a decent lead on the last day, then try to bowl South Africa out quickly."I definitely think we can have a crack at winning. The key is tomorrow morning to be nice and positive and see how we go leading up to lunch, and then give ourselves a couple of sessions to be able to bowl south Africa out," Clarke said. "The wicket is still pretty good for batting, though. We're going to need to maximize our time to have a chance of bowling South Africa out."Clarke is the leading run-scorer in Test cricket in 2012, reaching the 1,000 mark when he moved to 218 â at that moment, his Test average for the year was 111.11 in a span that includes a triple century and a double century against India.Until he smashed his career-high 329 not out against India in January, his highest test score had been 168. Since then, he also scored 210 in the same series against India.Clarke sprinkled 21 boundaries through his 350-ball stand and, apart from two loose shots immediately before and after he reached 50, was rarely troubled by the bowling attack rated as the best in the world.Cowan stroked 17 boundaries and had a few chances before he was dismissed, out of his ground when Steyn got a hand to a full-blooded Clarke drive and deflected the ball onto the stumps at the non-striker's end.It was the only wicket to fall on day four after Australia resumed on 111 for 3, making a mockery of predictions that the Gabba pitch would be fast and furious and be fruitful for the bowling attacks which combined featured five of the world's top nine pacemen.South Africa bowling coach and former Test batsman Allan Donald said he was surprised how little bounce and seam movement the pitch was allowing, considering its reputation, and said it was a long, hard slog for the bowlers."At the end of the day it took its toll a little bit," he said. "Everyone is feeling a bit (tired) right now."Clarke joined opener Cowan late Sunday after Ponting was dismissed for a duck and their nearly six-hour stand became the country's highest fourth-wicket partnership in a Test at the Gabba, surpassing the 245 by Mike Hussey and Clarke in 2007-2008 against Sri Lanka, and a record for that wicket against South Africa.The Clarke-Hussey combination resumed after the tea break, plundering the tiring South Africa attack. Hussey hit 12 boundaries and faced only 109 balls for his 86 in a partnership that was generating a run a minute.Cowan had a couple of chances, including one when he could have been run out on 106 after being sent back by Clarke when attempting a quick single and another on 123 when he hooked Rory Kleinveldt to deep fine leg, where Steyn and Jacques Rudolph got in the way of each other and put down a routine outfield catch.After proving he could convert a good start into a century and gave Clarke a gesture to signal no hard feelings when he was finally run out after 388 minutes at the crease.The 30-year-old lefthander's place in the test lineup was under scrutiny coming into the series, but his assured innings â particularly after the failures of David Warner (4), Rob Quiney (9) and former captain Ricky Ponting (0) on Sunday evening â will help secure his spot at the top of the order.His previous high score in seven tests was 74 against India in January, one of his three Test half centuries."I want to be playing cricket for a long time for Australia (and) there's only one way to do that," Cowan said. "There's only one currency, and that's scoring runs. I'm happy to be judged by the runs I score, and that's the bottom line."