Serena Williams: I Regret US Open Outburst

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.