Johannesburg, Feb. 25: Australian vice-captain Michael Clarke knows that he is racing against the clock to prove his fitness for the first Test against South Africa.

The Age quoted Clarke as saying last night that he needed to power through Australia''s final two training sessions to be seen as a guaranteed starter at the Wanderers Ground tomorrow.

He was Australia''s leading batsman against South Africa in the recent 2-1 series defeat and losing him would be a savage blow to the tourists.

"I think I''ll be OK. But I''ve got a big couple of days, today and tomorrow. It''s certainly improving and I''m confident I''ll pull up all right for the Test," Clarke said.

"Any time you''re not playing in the games, like the one we had last weekend, or any time you''re not able to train 100 per cent, there''s a few little nerves about if you''re going to be OK. With the physio we''ve got here and the fitness trainer we have here, I''m in the best possible hands. They take it personally if they can''t get someone on the park," he added.

Clarke has been nursing back problems since he was 14.

"I''ve been going through this since I was a kid. It''s not like I''m not used to it. I know I have to do a lot of strength work, core work, rehab, swimming, stretching. I know my body well enough now to know exactly what I have to do to stay on the park, " he said.

Asked if he would have until the morning of the match to prove his fitness, Clarke replied: "I hope so." (ANI)