Mitchell Johnson is like a wind-up toy. He'll tear in at the same speed all day but he has to be pointing in the right direction when the string is pulled. If he's slightly off line he might go flying off the table. If he's on the right path he could run through any obstacle in his way. With help from Dennis Lillee, Johnson set himself on the straightest course possible at the WACA and demolished South Africa's batting line-up and their spirits with a brilliant late spell of 5 for 2 from 21 balls.

It was one of the most devastating Test spells in recent memory and it brought back memories of some of the finest WACA fast-bowling efforts. Curtly Ambrose crashed through Australia at the same venue in 1992-93 with a spell of 7 for 1 that set up Australia's most recent series loss at home and Glenn McGrath mauled Pakistan in Perth in 2004-05 with 8 for 24. But the fast man most associated with the venue is the local bowler Lillee, who is now the president of the state's cricket association.

Lillee also happens to be the man who discovered Johnson as a 17-year-old and called him a once-in-a-generation bowler. Fittingly, when Johnson left his trail of destruction on the second day he did it while running in from the end of the WACA dominated by the Lillee-Marsh Stand. Johnson sought Lillee's counsel in the lead-up to the game and the advice was short and to the point. "He said at the WACA you probably want to be a little bit straighter," Johnson said, "try and get the guys to play a lot more."

It took a little while for the technique to kick in after Johnson began the day with a few of his familiar sprays well wide of the batsmen. He readjusted his radar and picked up Neil McKenzie and Graeme Smith, and late in the day he displayed the precision of a master craftsman. The aim was to use the Fremantle Doctor, the afternoon sea breeze in Perth, to help bring the ball back into the right-handers and then slip in one of his more standard deliveries angled across the bat.

It worked. AB de Villiers and Jacques Kallis, both of whom had scored 63 and appeared to be guiding South Africa into an advantageous position, succumbed to the trap and edged behind to fall to sharp catches from Brad Haddin. The collapse that followed left South Africa at 8 for 243 at stumps and handed Australia the momentum they had been missing all day.

The other factor that worked in Johnson's favour was aggression. Once the two established men were gone, he made the debutant JP Duminy uncomfortable with a bouncer that popped up off his gloves and then Morne Morkel succumbed to the bouncer/slower-ball combination. The fire is a relatively new part of Johnson's psyche - in his younger days he seemed too nice to be a fast bowler - and he said he had made a conscious effort to become more hostile.

In an attack missing Stuart Clark, still trying to find its way in the post-McGrath and Warne era and fighting off a challenge from the world's No. 2 team, Johnson's form spike is superbly timed

"[I'm trying to] bowl that bouncer, bowl the ones up into their ribs and then try and get that ball to cross the right-handers or other way round for the left-handers," Johnson said. "I'm trying to be a little bit more aggressive, be a bit more in their face, be a bit more confident."

It's easier when you're pumped up after a couple of wickets. Johnson's rampage lasted 33 minutes and if Australia's first half hour on the opening day, when they lost 3 for 15, harmed their plans then South Africa's finish to the second day almost killed theirs. Johnson's demolition ended with the penultimate over of the afternoon and while South Africa were pleased to have a night's rest that might quell Australia's spark, Johnson had pushed himself as far as he could and suffered cramps when he bowled his final ball.

He finished the day with a career-best 7 for 42 and he provided an irresistible period for the spectators who had stuck around on a day of South African grit. Johnson moved to Western Australia from Brisbane during the off-season because of the lure of his Perth-based girlfriend. He has not yet played for his new state but by the end of the day the fans were claiming him as their new "local" hero.

"I was copping a bit of flak earlier, they were still calling me a Queenslander," Johnson said. "I just let that run off my back and I started getting a few wickets and the crowd were right into it. It's a crazy feeling when the crowd's standing up and applauding you and you almost don't know what to do. But you just appreciate it and take it all in."

It was his first game at his new home ground since making the switch. He ended up with his second five-wicket haul in Tests after collecting his first at his real home venue, the Gabba, less than a month ago. Since he completed a disappointing tour of the West Indies, Johnson has picked up 34 Test wickets at 21.08. It is a period that has thrilled his mentor Lillee, who said: "It's very pleasing to see the coming of age of any fast bowler, and Mitchell on his new home ground stepped up to the mark today to show that he has definitely come of age as a Test bowler."

In an attack missing Stuart Clark, still trying to find its way in the post-McGrath and Warne era and fighting off a challenge from the world's No. 2 team, Johnson's form spike is superbly timed. If he keeps pointing in the right direction whenever that string is pulled, his master Ricky Ponting will have a big smile on his face. And it's not even Christmas yet.