Pretoria - Australian paceman Mitchell Johnson sounded a warning over his form on Thursday after giving South Africa the kind of treatment England couldn't handle in their Ashes whitewash.

Johnson took 4-51, including three for 10 in his four-over opening spell, to reduce South Africa to 140-6 at the end of the second day of the first Test at SuperSport Park.

“I started better today than at the start of the Ashes in Brisbane,” the left-armer told reporters after ripping through the home side's top order.

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

Johnson removed captain Graeme Smith, Alviro Petersen, Faf du Plessis and Ryan McLaren as South Africa closed 257 runs behind Australia's first-innings 397.

The Queenslander's anticipated contest with Smith - whose hand he has previously twice broken with nasty deliveries - lasted just two balls, as Smith flayed at a bouncer and was caught for 10.

Johnson said he had a plan to bowl a little wider at the crease and use the angle.

“It is nice when a plan comes off,” the 32-year-old said.

“I think that (the broken hands) is definitely in the back of his (Smith's) mind, but it was just one of those ball that took off.

“I think I'll try and keep bowling like that to him and try to keep producing those kind of balls.

“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 said he was enjoying his role as the team's aggressor, working not only on the batsmen but also exchanging insults with the crowd. “I'm trying to be as intimidating as possible,” he said.

Johnson took 37 England wickets in the five-Test Ashes series at an average of less than 14. – Reuters