Djokovic, a three-times champion at Flushing Meadows, was never really pushed by the German, who has taken just one set off the Serb in their five career meetings.

The only drama came early on when Struff had break point opportunities but could not convert against Djokovic’s serve in the first game.

The players stayed on serve until Struff sent a forehand wide on break point to give Djokovic a 5-3 lead. The Serb held to love the next game after firing an unreturnable serve to take the first set and coasted the rest of the way.

“It was a very, very good performance from my side,” Djokovic said after recording his 600th career win on hard courts.

“I managed to read his serve well. Made one necessary break in the first set, I faced one or two break points in the opening game.

“Obviously the trajectory of the match can go differently if you lose your serve against a big guy like Struff, who serves really well and has a very aggressive, powerful game from the back of the court,” said the Serb, who is looking for an 18th Grand Slam title.

“But I moved well and after the first set, the second and third was a really great feeling on the court.”

Djokovic and Marin Cilic, who won the US Open in 2014, are the only two men remaining in the draw who have won a Grand Slam tournament.

Djokovic will be a heavy favourite when he faces 20th seed Pablo Carreno Busta on Spain in the fourth round on Sunday.