Novak Djokovic battles past debutant Dominik Koepfer 6-4, 4-6, 6-3 to reach the semi-finals of the Rome Masters on Saturday. The 17-time Grand Slam champion was too good for the German qualifier who knocked out tournament surprise package Lorenzo Musetti in straight sets 6-4, 6-0 on Friday. The Serb, who beat compatriot Filip Krajinovic on Friday to secure his place in the quarter-finals for the 14th year in a row, stormed into a 4-0 lead before clinching the first set. Koepfer showed resilience in the second, fighting back from a break down to take the second 6-4. But after going a break up in the third and ninth games of the final set, the world number one marched into the semi-finals for the seventh year in a row where he will take on 21-year-old Norwegian Casper Ruud. Novak Djokovic's frustration boiled over once again as he smashed his racket in a fit of rage. The top-ranked Serb was broken to love in the sixth game of the second set following which he threw his racket on the ground, drawing a warning from the chair umpire. "Well, let me tell you it's not the first nor the last racket that I'll break in my career," Djokovic, 33, told reporters. "I have done it before, I'll probably do it again. I don't want to do it, but when it comes, it happens. "That's how I guess I release sometimes my anger. And it's definitely not the best message out there, especially for the young tennis players looking at me. He said: "I don't encourage that, definitely. But, look, we're all people. We all do our best. There were times and periods when I don't do that, and there are periods when I do." The Serb was disqualified from the US Open after inadvertently hitting a ball into a line judge's throat during his fourth-round match against Pablo Carreno Busta. Djokovic, who apologised at the time and vowed to take away valuable lessons from the incident, reiterated that he was working on his "mental and emotional health" to try and help keep a lid on his emotions. "It's always been part of my, I guess, training and recovery, as well, developing strong character and understanding myself on different levels, holistic approach to life," the 17-time Slam champion said. "That's just me. Of course I'm not perfect. I'm doing my best."