Cricket: New ball key for Australia - Siddle

Australian bowler Peter Siddle has taken the only two South African wickets to fall so far in the second innings. Photo / AP

DURBAN - Paceman Peter Siddle says Australia will put faith in the second new ball to give them the breakthroughs they need on the final day of the second test against South Africa in Durban.

Chasing a world-record target of 546 to win and with captain Graeme Smith unlikely to bat because of a fractured little finger, South Africa fought on well to stumps on day four, reaching 244 for two.

Jacques Kallis is 84 not out and has added 164 for the third wicket with A.B. De Villiers, who is unbeaten on 68.

Phillip Hughes was finally dismissed for 160, his second century of match, before Australia declared at 331 for five after 7.4 overs at the Kingsmead ground on Monday morning (local time).

Trailing 1-0 in the three-match series, the South Africans weren't daunted by the task of batting out almost two full days and Australia will need to grab their chances on day five.

"Definitely it's going to be important," Siddle said of the second new ball.

"It has played a big part in the whole match so far, the new ball.

"We'll see how we go tomorrow morning if we take it straight away. I'd be thinking personally that we'd probably take it and hopefully get a break through."

Siddle had Neil McKenzie caught behind for 31 at 1-63 and next ball Kallis edged to first slip where Marcus North dropped a difficult chance.

Siddle struck again six overs later when Hashim Amla was caught at second slip by Ricky Ponting for 43.

The Proteas were forced to promote No 3 batsman Amla to the opening role in place of Smith.

Kallis was dropped again on 33 by wicketkeeper Brad Haddin off Siddle's bowling, although replays suggested doubt over whether the batsman edged the ball.

Earlier, Hughes was a young man in a hurry, scurrying through for a cheeky bye here and there and smashing short balls through cover.

The 20-year-old's six-hour knock ended when he was caught at third man after hitting 15 fours and three sixes. Michael Clarke was 23 not out and North made a duck.

Hughes became the youngest player to score twin centuries in a test.

The NSW left-hander also became the second-highest scorer in his first two tests with 0, 75, 115 and 160 for a total of 350 runs at an average of 87.50.

England's Reginald "Tip" Foster, who made his debut in Sydney in 1903, hit 355 runs in his first two matches.

- AAP