Karthik finished on 146 not out and together with India captain Mahendra Singh Dhoni (91) shared a fifth-wicket stand of 211 that turned the innings on its head.
Regarded as the reserve wicket-keeper behind Dhoni, Karthik has almost certainly played his way into the side as a batsman on the back of this 140-ball innings featuring a six and 17 fours.
This was Karthik's second century, albeit in warm-up matches, after his unbeaten 106 against Sri Lanka on June 1.
"I've been hitting the ball really well," Karthik said at the innings break. "When you come in at six, it's important to assess the situation. When you have Dhoni around it's important to rotate the strike.
"It was a good bowling attack, and it feels good to get some runs. I have had a good IPL and it's important to keeping the same momentum from that and it's important to keep going from there. I've always been very comfortable playing in the middle order so hopefully that will continue."
Dhoni produced a typically quickfire innings, facing 77 balls including six fours and four sixes after left-armers Mitchell Starc and Mitchell Johnson, together with Clint McKay, had run through the top-order.
Champions Trophy holders Australia, however, were without captain Michael Clarke because of a recurrence of a longstanding back injury which has made him doubtful for the side's tournament opener against England at Edgbaston on Saturday.
India face South Africa in the first match of the Champions Trophy proper at Cardiff on Thursday.
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