The new ball will hold the key to Australia's [Images] chances of clinching the Test series against South Africa [Images] on Tuesday, said pace bowler Peter Siddle [Images].

The home team, chasing an improbable victory target of 546, will resume on 244 for two on the fifth and final day of the second Test.

Jacques Kallis [Images] (84 not out) and AB de Villiers [Images] (68 not out) have shared an unbroken third-wicket stand of 164 but the second new ball will be available to Australia's bowlers first thing in the morning.

"The new ball will definitely be crucial, we've seen it play a big part in the whole match," Siddle told reporters after he removed Hashim Amla [Images] for 43 and Neil McKenzie [Images] for 31 on Monday.

"If we use the new ball ... well enough then we like our chances of winning."

Siddle also said Australia, 1-0 up in the three-match series, would need to stay patient throughout the day.

"It won't be easy, it will push into the final session for sure," he said.

"It will be all about patience, working the batsmen over and sticking to the plan which is to put pressure on the batsmen."

FIRST SESSION

Amla, who opened the batting for South Africa in the absence of the injured Graeme Smith [Images], agreed the new ball would be important to the outcome.

"A large amount depends on the first session," said Amla. "It will be an intense first hour and we have no doubts they will come very hard at us.

"The big deciding factor will be that first session with the new ball."

However, Amla was confident the experienced Kallis and De Villiers could see South Africa through to a draw.

"Jacques has shown he can bat for six hours for the last 12 years and all the batsmen have done it in the last two years," said the makeshift opener.

"The pitch does seem a bit variable in terms of pace and bounce, some balls are shooting up and others keeping low, but by and large it has not reacted too unfavourably.

"There are cracks straight down the pitch and if they open up then it will be more difficult."