Australia is assured of its first ever Olympic baseball medal following a shock win over Japan in Athens.

In the nation's finest moment in the sport, the unheralded Australians defeated the Japanese 1-0 in the Olympic semi-final to advance to the gold medal game.

The Australians are guaranteed at least a silver medal from Wednesday's final, to be played against the winner of the Cuba-Canada match later Tuesday local time.

Starting pitcher Chris Oxspring, first baseman Brendan Kingman, centre fielder Brett Roneberg and relief pitcher Jeff Williams all took turns at assuming hero status as Australia powered its way to an upset.

Oxspring was outstanding on the mound, shutting down Japan's potent batting and limiting it to just five hits.

Then in the sixth inning, with Roneberg on third base and two outs, Kingman stepped up and belted the ball through centre field for a double to bring Roneberg home for the winning run.

Australia then had to survive a nervous seventh inning, with two errors in the field allowing Japan's first baseman Michihiro Ogasawara to advance to third base and within sight of a levelling run.

But then coach Jon Deeble replaced the tired Oxspring with relief pitcher Jeff Williams, who plays professionally in Japan, to pitch against his Hanshin Tigers teammate Atsushi Fujimoto.

It proved a master stroke, with Williams mopping up Fujimoto for the final out Australia needed to close the seventh inning when the Japanese batter popping a fly ball to shortstop Rodney van Buizen.

Then the 32-year-old Williams pitched nervelessly in the eighth and ninth innings, with the Japanese unable to register a hit on his pitching.

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