Do your homework, kids. World No. 1 Novak Djokovic showed how important that is in his third-round victory Thursday against John Isner at the Rolex Shanghai Masters.

Isner reached the 1,000-ace mark for the sixth time in his career with his second ace of the match. But it was the number of aces top-seeded Djokovic allowed the American to blast by him that was the story of the match.

Djokovic matched Isner’s nine aces in his 7-5, 6-3 victory against the World No. 17 to reach the Shanghai quarter-finals.

“It's always a big challenge returning the serve of Isner. He's got one of the biggest serves of all-time. He's one of the tallest guys ever to play tennis. Obviously with that height, [his] serve is a huge weapon and huge advantage,” Djokovic said. “I managed to read his serve and find a good position on the return [at the] end of the first set and also [at the] beginning of the second.”

Djokovic now leads Isner 10-2 in their FedEx ATP Head2Head series, and in seven of their 10 matches outside of Davis Cup, Isner has failed to strike at least 10 aces. The American has averaged more than 18 aces per match in his career.

“Of course I do my homework and I talk with my coaches and understand what his pattern is, what his favourite angles or serves are, if there are some,” Djokovic said. “He can hit any serve at any angle with any pace and any rotation.”

That homework has paid off for Djokovic, who won 42 per cent of his first-serve return points against Isner in Shanghai. Isner won an average of 83 per cent of his first-serve points in his two victories before falling to Djokovic.

“I expected him to go more for the second serves, which was the case, but I didn't expect him to not make a single double fault and really hit every second serve over 200. That was really impressive,” said Djokovic, who only won 27 per cent of his second-serve return points. “But still, I was very pleased with the way I returned. I returned a lot of balls back and just made him play.”

Djokovic has won seven consecutive matches to start the Asian Swing, including his 76th tour-level title at the Rakuten Japan Open Tennis Championships last week. The Serbian has not dropped a set in those triumphs. After retiring in the fourth round of the US Open due to a shoulder injury, Djokovic didn’t feel there was a glaring hole in his game he had to address.

“There are always some fundamental things to work on in the game that keep on repeating. It's necessary to train in order to feel well, to feel confident, to feel that you're striking the ball nicely,” Djokovic said. “The main priority was for me to be healthy and try to rehab, go through rehabilitation process of the shoulder in the best possible way, because I didn't know whether I'm going to be able to play Asia or not. I'm just happy that my shoulder has been holding on and not causing me any pain.”