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Shin completed the four-round LPGA tournament at Royal Canberra in 18-under 274, two shots clear of world No.1 Tseng and four shots ahead of Ko (278).
Former world No.1 Shin was hot out of the gates today, birdieing the first hole par 5 as her overnight co-leader Ko double-bogeyed.
A bogey on the second hole by 15-year-old Ko then delivered Shin a sizeable four-stroke advantage.
But Ko fought back, birdieing consecutive holes while Shin slipped a shot with a bogey on the fifth after running into bunker trouble.
Shin then dropped another stroke on the 12th to fall equal with Ko and a blistering Tseng soon made up her eight-stroke overnight deficit to make it a three-way tie.
Under pressure and deep in the rough to the left of the 14th green, Shin then chipped in for a birdie to snare the lead.
She followed that up with another birdie the next hole to all but seize the championship.
Queenslander Katherine Hull-Kirk was the best of the Australians, finishing equal 8th on 281.