Five-time champion Djokovic opened his campaign with a straight-sets win over the powerful John Isner and he consolidated his place at the top of Group Guga Kuerten with an unwavering triumph against Zverev.

The world number one's place in the semi-finals will be secured if Isner overcomes Marin Cilic or the Croatian wins in three sets.

Djokovic said before the match he expected Zverev to play at a higher level than their previous meeting – a 6-2 6-1 win that only took an hour and saw the Serbian break four times without facing a break point himself.

While Zverev did provide a far greater test in the opening set, the 21 year-old was unable to keep his head in the match after falling behind and made copious unforced errors as Djokovic charged on to a commanding victory.

Neither player gave anything away in the opening stages and it was not until the ninth game that either was able to engineer a break point, Zverev having benefited from a net cord that sent his forehand looping over Djokovic.

The German was unable to take his two chances to move in front as Djokovic held on, and the top seed was ahead when, on his third set point, the German frustratingly double faulted after his excellent start.

Although Zverev showed great character to hold on to his first service game in the second set, it proved to be the last one he would win during the match.

A wayward shot from the German went long and gave Djokovic two break points, before a botched backhand put the 31-year-old 3-1 up.

Zverev's poor volley at the net enabled Djokovic to take the first of three break points in game six and victory was complete after a long forehand from the world number five.