Novak Djokovic did not think he would be healthy enough to play in Asia, after withdrawing from the US Open with a left shoulder injury.

But the world No. 1 appears to have rediscovered his form, winning the Japan Open in Tokyo without dropping a set last week.

He also won his first two matches at the ongoing Rolex Shanghai Masters, where he has four titles - in 2012, 2013, 2015 and last year.

After defeating John Isner 7-5, 6-3 yesterday to book a quarter-final place, the Serb said the key to victory was reading the American's serve well and finding a good position on the return to break his opponent's serve in each set.

"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)," added the 32-year-old of his 2.08m opponent.

"I kind of held my serve, I thought it was one of the best serving matches I had lately."

Both men served nine aces each, but Djokovic performed better on his first serves. He won 88 per cent of his first-serve points while Isner won 58 per cent, and the Serb got 79 per cent of his first serves in (versus the American's 76 per cent).

The 16-time Grand Slam champion said he did not work on any specific area of his game between the US Open and Tokyo, as his main priority was to stay healthy and rehabilitate his shoulder.

"There are always some fundamental things to work on... it's necessary to train in order to feel well, to feel confident, to feel that you're striking the ball nicely," said Djokovic, who plays world No. 7 Stefanos Tsitsipas today.

He acknowledged the pressure that comes with doing well but added: "When you feel pressure, you're doing well. It's something that is inevitable, so you might as well embrace it and accept it.

"Because of the history that I had in this tournament and in general in the Asian swing and lots of success and lots of matches won in my career, I actually feel less pressure because I'm more confident that I can perform my best."

Earlier, Russian Daniil Medvedev overcame a 5-2 deficit and saved five set points in the first set en route to beating Canadian qualifier Vasek Pospisil 7-6 (9-7), 7-5 in 2hr 5min on a warm and sunny day at Qi Zhong Stadium.

The 23-year-old said: "The sun was disturbing a lot (on) both sides... you go into the sun and the other (player) is in the shadow so, when he hits the ball, you cannot really see it.

"Many times (it's) like this when you play around this hour, you need to be ready for this and need to win like this."

Two-time Shanghai champion Roger Federer also saved five set points in the first set in his 7-6 (9-7), 6-4 win over Belgian David Goffin. His next opponent is Alexander Zverev, who ousted Andrey Rublev 6-0, 7-6 (7-4).

World No. 4 Medvedev will face Italy's Fabio Fognini, who insisted he was not upset at being told to "shut up" by Andy Murray during their second-round tie on Tuesday.

Tempers flared after former world No. 1 Murray accused him of shouting during a point, and both men then had a testy exchange on the changeover during which Murray told the Italian to "shut up".

Fognini, who beat Russian Karen Khachanov 6-3, 7-5 yesterday, said he and Murray "talked normally" after their match.

"I've known him since we were 14 and that's it. That kind of thing for me stays on court. He has his character, I have mine," he added.

"We know each other really well. Inside the court we are almost the same because most of the time I'm complaining and he's complaining. But that's part of our job."

SHANGHAI MASTERS

Q-finals: StarHub Ch201, 12.30pm