Mitchell Johnson has revealed his delight following the "great start" he made with the ball in the first Test against South Africa.

Australia's fast bowler impressed on day two in Centurion, taking three early wickets before finishing the day with 4-51 to help his side reduce the hosts to 140-6.

Johnson claimed after play finished that he is in better form now than at the beginning of the Ashes series, in which he took 37 English wickets.

"I started better today than at the start of the Ashes in Brisbane," Johnson admitted to reporters.

"To come out and get three wickets quickly is definitely a great start."

"When I was batting I could sense the extra bounce in the wicket, so when I started bowling I tried to hit the deck as hard as I could. I didn't feel the wicket was swinging but there were cracks to work with."

Johnson was recently called the best fast bowler in the world by Australian coach Craig McDermott.