Less than two weeks after getting defaulted from the US Open, Novak Djokovic lost his cool again midway through a 6-3 4-6 6-3 win over German qualifier Dominik Koepfer in the Italian Open quarter-finals on Saturday.

When Djokovic was broken at love to even the second set at 3-3, he slammed his racket to the red clay in anger.

With the frame broken and the strings all mangled, Djokovic was forced to get a new racket and received a warning from the chair umpire.

Djokovic had already appeared frustrated the previous game, when he glared toward the umpire following a couple of overrules and a point that was ordered to be replayed.

The top-ranked Djokovic had said Monday that he learned "a big lesson" after he was thrown out of the U.S. Open for unintentionally hitting a line judge in the throat with a ball in a fit of anger.

Djokovic also acknowledged then "that I have outbursts and this is kind of the personality and the player that I have always been."

The 97th-ranked Koepfer, who screamed at himself in frustration throughout the match, was also warned for misbehaviour early in the third set.

Aiming for his fifth title in Rome, Djokovic's semi-final opponent will be Casper Ruud, who eliminated local favourite Matteo Berrettini 4-6 6-3 7-6 (7-5) in a match that lasted almost three hours.

Ruud is the first Norwegian to reach the semifinals of a Masters 1000 tournament.