The New Zealand Black Sox are in prime position at the Challenge Cup softball tournament in Auckland after a stunning finish in pool play tonight.

Three wins today saw them automatically qualify for tomorrow's final, where they will face either Argentina or Japan.

It was Argentina who produced a classic in the late game, with the Black Sox coming back from 6-1 down in the bottom of the final inning to win 8-6 on a walk-off grand slam by Isaac Fletcher.

In a marathon clash, the Black Sox looked dead until the drama-filled bottom of the final inning, which saw three singles, an error, two walks, an illegal pitch, a hit by pitch which caused a mini-brawl and lengthy delay, and finally the game-ending grand slam.

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That result left the Black Sox with a 9-1 record in pool play, with their only loss coming on Friday where they fell to a 2-1 defeat to Japan.

They started Saturday with a comprehensive 8-1 victory via mercy rule over the Czech Republic, but were given a tougher contest later in the day against Australia, grinding out a 5-4 win in a seven inning battle.

Home runs to captain Thomas Enoka and third baseman Joshua Harbrow gave the Black Sox an early lead, and although Australia pegged one back, the Kiwis scored three in the top of the fifth to build a 5-1 buffer.

Australia again stayed in touch by reducing the margin to just one by the bottom of the sixth, but the Black Sox held steady in the final inning.

The Black Sox will be tested in the final today, with the winner of the playoff between Japan and Argentina - ranked second and third in the world respectively - sure to provide a challenge.

The 2 v 3 playoff game starts at 10.30am, with the final getting underway at 2.30pm.