Embattled Kangaroos must win heavy to go through on run rate while any victory will do for Sri Lanka.
A tall order awaits Clarke's team as Australia take on Sri Lanka
The batsman is yet to play in the Champions Trophy as he struggles with a back injury, prompting people to question his condition for the first Test against England at Trent Bridge on July 10.
But ahead of the clash with Sri Lanka, which the Kangaroos must win by a big margin to have any chance of progress to the semi-finals, Clarke revealed he is still taking things day-to-day when it comes to his fitness.
"I don't know at this stage," he said when asked about his injury yesterday. "I'll have to train today with the boys and see how I pull up tomorrow. But I'm hopeful, there's no doubt about that.
"I really need to wait and see how I pull up tomorrow morning after training again today."
Clarke, 32, has suffered from regular back problems during his career and most recently suffered a relapse during the 4-0 series defeat in India.
He was also absent from the Champions Trophy warm-up games against West Indies and India and has been receiving specialist treatment in London in a bid to regain his fitness.
He said he is not worried about his prospects of missing games against England, however, saying: "I'm confident, but I've been confident that I'd play every game in this series, as well. So, you're probably asking the wrong guy.
"Look, it's just daily maintenance, daily physio, and I guess I'm at a stage now where I'm slowly getting back into my cricket training.
"I've been at training the last two days. My back is certainly improving, which is a positive. I just need to push it again today and see how far I can get, and if I pull up all right tomorrow, then hopefully the Australian cricketers will allow me to take the field tomorrow."
The Australians have faced a hectic build-up to the match, with David Warner being suspended following his bar-room clash with England batsman Joe Root in Birmingham.
That, combined with a run of indifferent 50-over form, will have Sri Lanka as favourites going into today's match but captain Angelo Mathews is refusing to take victory for granted.
"The mood in the camp is very good," he said. "We're feeling very confident, but I think tomorrow it's a fresh game.
"We must start from zero once again. So we need to be up for the game against Australia because we all know that Aussies play well.
"They've not been in the best of form, but still, we can't be complacent against them because they've got some really good players, so we have to be on our guard all the time."
After England beat New Zealand last night, Sri Lanka were left in a must-win situation. If weather does not allow play, a point added to their seven-wicket win over England will not be enough.
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