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Sydney Sweeney was boots on the ground at Dodger Stadium Tuesday night for game 4 of the World Series, and she had some company ... taking in the action alongside Christy Martin !

Sweeney -- who portrays Martin in the soon-to-be-released biopic, Christy -- shared a selfie of the two from Chavez Ravine, and the women were nothing but smiles as they settled in to watch Shohei Ohtani take the mound.

It's not the first time we've seen the duo together; they hit the red carpet for the premiere of the boxing flick last month. While that was for work ... tonight's appearance shows the two have developed a strong bond.

Sweeney also got viewers ready to go for the big Fall Classic game following the 18-inning marathon thriller Monday night ... starring in a minute-lon

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