Tour match, Taunton: Somerset 345-6 (46.5 overs) beat Australia 342-5 (50 overs) by four wickets
Smith hammers the ball away to the mid-wicket boundary
Graeme Smith and Sanath Jayasuriya flayed Australia's bowlers to inspire Somerset to a magnificent win in front of a sell-out crowd at Taunton.
Set a target of 343, the county side reached 345-6 with 19 balls to spare after Smith (108) and Jayasuriya (101) had shared a first-wicket stand of 197.
The defeat was Australia's second loss in three days, having been beaten by 100 runs by England in a Twenty20 game.
James Hildreth hit the winning boundary to finish on 38 not out.
Australia, who play their first match in the NatWest Series when they take on Bangladesh on Saturday, will be concerned about the form of their support bowlers.
Michael Kasprowicz conceded 89 from eight overs while all-rounder Shane Watson went at a rate of 14 an over in his first three and finished with figures of 1-72.
Everything went according to plan earlier in the day as the tourists made the most of a good wicket at the County Ground to pile up a total of 342-5.
Matthew Hayden struck the first ball of the match past extra cover for four and raced to a 39-ball half century before retiring on 76.
Skipper Ricky Ponting, who had a spell with Somerset last summer, followed suit after hitting three sixes in his innings of 80, putting on 119 with Damien Martyn (44).
Michael Clarke (63 not out) and Mike Hussey (51) provided the required acceleration at the end of their innings, adding 99 in 12 overs.
Australia's total looked well out of reach, but South Africa skipper Smith and his Sri Lankan partner showed no respect to their attack.
They took 26 off Brett Lee's first four overs with the new ball before he eventually went off with a sore shoulder.
The tourists, who lost a one-day game to Middlesex at a similar stage of their 2001 tour, were powerless to halt their momentum as Smith raced to his century off 68 balls, with 17 fours.
Jayasuriya struck three sixes and was only nine balls slower in reaching three figures.
Smith was eventually stumped off left-arm spinner Clarke while Jayasuriya was caught by Hussey at long leg in Glenn McGrath's second spell with the total on 231.
McGrath also bowled John Francis behind his legs for 23 and Watson saved face with the vital wicket of Ian Blackwell, who made 25.
Hussey nipped out Keith Parsons and Matthew Wood but Hildreth, who has been out of form so far this season, and reserve keeper Carl Gazzard saw their side home.
Gazzard made an unbeaten 21 off 12 balls before Hildreth sealed victory with a lucky boundary off an inside edge.