It was far from plain sailing for the 33-year-old Serb though and there was an anxious moment when a ball flew off his racket during the first set and hit a line judge in the head, reviving memories of his default at the U.S. Open. Djokovic had conceded no sets and only 15 games in his first three rounds but the powerful 15th seed Khachanov provided a much better barometer of his form in his quest for a second French Open and 18th major title. “It was definitely closer than the score suggests,” Djokovic, who has bounced back strongly from the ignominy of being defaulted at the U.S. Open for inadvertently striking a female line judge in the throat with a ball, said. “It was quite even all three sets and I managed to break his resistance towards the end of each set but it was a close one definitely. I’m happy the way I closed it out.” ADVERTISEMENT Djokovic is through to his 47th Grand Slam quarter-final and 11th in succession at Roland Garros and will face either German qualifier Daniel Altmaier or 17th seed Pablo Carreno Busta of Spain. Carreno Busta was the player who benefitted from Djokovic’s indiscretion in the U.S. Open fourth round. Djokovic faced stiff resistance in the first set and it was while pushing for a break in the eighth game that he stretched to return a wide serve and the ball flew off his frame and into the temple of a line judge from close range. This time it was a completely innocent accident and Djokovic apologised and moments later broke serve after a gruelling 10-minute game. Russian Khachanov broke back though immediately with a dipping forehand before tamely handing Djokovic the set in the next game with a double fault. ADVERTISEMENT Djokovic relied heavily on his drop shot again as he moved 4-2 ahead in the second but Khachanov then had four break points but failed to convert any of them. Khachanov survived four set points at 2-5 before Djokovic calmly held to move two sets clear. Djokovic could never totally relax though and from 2-0 ahead in the third set, he lost three consecutive games and saved two break points to avoid going 4-2 down. The Serb is never more dangerous than when his back is against the wall though and he regained control to ease home and claim a 35th win in 36 matches this season -- the one blot being his default in New York. Reporting by Martyn Herman, editing by Ed Osmond and Pritha Sarkar