Serena Williams is telling friends ... she's sorry about her outburst during the women's final at the US Open yesterday -- when she berated a referee for penalizing her at a crucial point in the match.
Sources extremely close to Serena tell TMZ ... the tennis star feels like she got lost in the heat of the moment -- and simply believes her emotions got the better of her.
Serena was fined $2,000 for the outburst -- but sources tell us, the tennis star is less concerned about the money than she is about getting her game back on track.
Calls to Serena's rep weren't returned.
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