Top men's seed Novak Djokovic dropped a set before fighting off German qualifier Dominik Koepfer to reach the semi-finals of the Italian Open on Saturday.

The world number one needed more than two hours to see of the 97th-ranked German 6-3, 4-6, 6-3 in the final warm-up tournament on clay before the French Open.

Two weeks after his US Open default for accidently hitting a line judge with ball, the Serb again showed signs of frustration, throwing his racket after a lost service game, with shouts of anger resounding in the silence of the empty Central Court of Foro Italico. 

Djokovic was broken four times before advancing to his 11th semi-final in Rome, where he reached the final nine times and won four titles.

"Credit to (Dominik) for fighting back, but I have myself to blame for putting myself in a position to play a third set," said Djokovic. 

"I was a set and a break up and everything was looking great. I just wasn't managing to make that final shot, that final step to win in straights.”

The 33-year-old next plays Norway's Casper Ruud who earlier battled past Italian fourth seed Matteo Berrettini 4-6, 6-3, 7-6 (7/5).

"Clay is definitely Casper's preferred surface," said Djokovic. 

"This is where he feels most comfortable. It's semi-finals and it is anybody’s game. I'll do some homework and be ready for that one."

Djokovic is bidding for a record 36th Masters crown in the Italian Open, being currently tied on 35 with Spain's Rafael Nadal, the number two seed in Rome.