Ashes 2015: Mitchell Johnson among wickets for Australia

Mitchell Johnson has taken 72 of his 291 wickets in Tests against England

Mitchell Johnson took three wickets to provide a timely reminder of his abilities as Australia continued to dominate Kent in their tour match.

Johnson's Ashes spot is under threat after Mitchell Starc and Josh Hazlewood out-bowled him against West Indies, but he finished with 3-42.

Ex-England batsman Rob Key hit 87, but Kent slid to 203-5 at stumps.

Australia vice-captain Steve Smith had earlier retired on 111 as the tourists declared on 507-8.

Left-armer Johnson was England's nemesis in the Ashes whitewash down under in 2013-14, taking 37 wickets in five Tests.

But the performances of Hazlewood and Starc in the Caribbean, and the return of right-arm seamer Ryan Harris, left him with something to prove on another warm day in Canterbury.

It did not take long. At the end of a fiery first over, Kent opener Daniel Bell-Drummond was pinned lbw to give Johnson his first wicket of the tour with just his sixth ball of the day.

Riding their luck at times, Key and Joe Denly withstood the remainder of a testing opening spell from Johnson and Harris to put on 81 for the second wicket.

After Denly was dismissed by Peter Siddle for 36, Northeast and Key added a brisk 59, only for Northeast to glove the third ball of a new Johnson spell to wicketkeeper Brad Haddin.

Key, who played the last of his 15 Tests in January 2005, bludgeoned three fours in five balls off struggling leg-spinner Fawad Ahmed to move within sight of a century.

But his aggression proved his downfall, as he lobbed the last ball of the over to Shaun Marsh at midwicket.

Johnson showed his potency once again in clean bowling Ben Harmison for five, but a gutsy unbroken stand of 35 between Sam Billings and Adam Ball guided Kent, who were 140-2 at one point, to the close without further damage.

Earlier, Smith picked up where he left off on Thursday as he breezed to a century from 151 deliveries.

Two boundaries later, Test cricket's top-ranked batsman retired to give someone else a turn.

That someone was Mitchell Marsh, whose effortless strokeplay contrasted sharply with the travails of Shane Watson, his rival for an Ashes place.

Watson fell to a spectacular one-handed catch by Denly for 21, before Marsh had his off stump uprooted by Matt Hunn to end a 26-ball innings of 30.

Hunn, who plays most of his cricket for Sandwich Town in the Kent Premier League, picked up figures of 5-99 in his first Kent outing of the season.

The 21-year-old was then applauded from the field after Australia declared following quick runs for Haddin (35) and Johnson, who finished unbeaten on 32.

'I don't get paid enough to face Johnson'

Australia batsman Steve Smith responding to criticism from former England spinner Graeme Swann about weaknesses in his technique: "That does not really bother me. It is pre-Ashes and a little bit of banter.

"I am happy for him to say whatever he likes, and hopefully I can just continue to let my bat do the talking.

"You can expect that, trying to get into guy's heads. It has certainly happened before and will do again.

"For me, it is just about going out there and playing the type of game I have been over the past 12 to 18 months and continuing to score some runs."

Rob Key: "For a large part of that I thought 'there is no way I get paid enough to be facing Mitchell Johnson'.

"I do all right for myself, don't get me wrong [but] he is on a different level.

"The pace he has, he gets the odd one to swing. It doesn't matter which pitch you are on, that bloke is a serious bowler.

"He can intimidate you but he can also move the ball around a little bit. He's a proper, proper bowler."