MICHAEL Clarke faces a "sink-or-swim’’ challenge when he takes over from the injured Ricky Ponting as Australia captain in the Sydney Test tonight.

Clarke could hardly have inherited the role at a trickier time, when his own batting form has deserted him and Australia are in grave danger of losing the Ashes outright on home soil for the first time in 24 years.

His opposite number Andrew Strauss notes that Clarke has been groomed as Ponting's long-term successor for several years.

But it was not supposed to happen quite like this, with Ponting on the sidelines because of a broken finger - aggravated on the way to defeat against England in Melbourne last week - and Clarke somehow needing to revitalise an apparently flagging team.

"It is a big day for Michael Clarke coming in and captaining in a Test match,’’ said Strauss, who still recalls the shock to the system when he first took over in charge of England.

"Captaining in a Test is very different to captaining a first-class game.

"Michael Clarke has long been earmarked as a captain - and by all accounts tactically he is pretty good.

"As I found, though, you have to learn on the job - and you have to learn reasonably quickly. People sink or swim.’’ There will be no hiding place, with Australia needing to find inspiration from somewhere and millions waiting to pass judgment on Clarke's efforts.

"It is like your Test debut,’’ added Strauss, who made a century on that occasion himself.

"People either stand up and deliver, or it becomes difficult. Michael Clarke has delivered plenty of times with the bat, so he is in a position to do well.’’ In fact, Clarke has managed to pass 50 only once in seven Ashes innings this winter - and Strauss acknowledges it can be harder for a captain to get things right when he is out of form with bat or ball.

"It's certainly a lot tougher,’’ he said.

"If you've got the double whammy of the side not playing well and you're not playing well, there's almost not enough time in the day to think about your game and what you need to do in the team.

"It is tough - but it's always temporary. That's the reality. For good players, personal form is only going to desert you for a short amount of time - and usually you come out the other side, which is reassuring for all of us.’’ Ponting is expected to remain with the Australia team throughout the Test - a situation with which Clarke is comfortable and in which Strauss is simply not interested.

"It's just none of our business, and quite frankly we've got different things to worry about rather than what's happening in their dressing room,’’ he said.

Strauss senses, because England have already retained the Ashes, Australia may feel less inhibited than they have done in the other four Tests.

"There is an opportunity for them to play with a bit more freedom, so we have to be on our game.

"It can be a bit of a release, knowing you've got nothing to lose any more.

"I think we're quite conscious of Australia coming back at us pretty hard this week - and if there are any weaknesses we can expose, it's important we do that quickly and early.’’ It has been spelled out, ever since England opened up their unassailable 2-1 lead at the MCG, that they cannot afford to let their guard slip here.

But Strauss was not above repeating the message one last time, for the benefit of a team likely to be unchanged tomorrow following their Melbourne success.

"It's easy to take your foot off the pedal. That's something I'll desperately avoid.

"It's a challenge for us, and I'd be very disappointed if we didn't respond well to that challenge.’’ The majority of England's Test squad still have another month ahead of them in Australia, with seven one-day internationals to come.

They therefore should not be demob happy.

"That is one of the challenges on the last game of the tour, that you're thinking about getting on the plane and seeing your family and getting home,’’ added Strauss.

"We're not in that position at the moment. But there are other reasons for us to take the eye off the ball in this Test match, and it's important that we don't do that.

"If you look back on our record we've been very good at coming back from defeats, less good after wins.

"That's something we've got to put right.’’