(AP Photo/Seth Wenig)

ROME (AP) Novak Djokovic knows it isn't model behavior when he loses his cool on the tennis court.

Yet he just can't help himself.

Exactly two weeks after he was defaulted from the U.S. Open, and a day after he was warned by the chair umpire for breaking his racket in a fit of rage, Djokovic received an obscenity warning midway through a 7-5, 6-3 win over Casper Ruud in the Italian Open semifinals Sunday.

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