ATP Finals: Novak Djokovic through after dismantling Alexander Zverev

World No 1 seals his place in semi-finals after 6-4, 6-1 rout of young German Alexander Zverev

Serbia's Novak Djokovic in action against Germany's Alexander Zverev. Reuters

Novak Djokovic has 834 Tour match wins in his storied career but the latest, a 6-4, 6-1 rout of young German Alexander Zverev at the ATP Finals in London on Wednesday, will not be one for his scrapbook.

There were few moments to test the 02 Arena's crowd decibel-level monitor - a new gimmick this year - but despite not feeling well, Djokovic had too much guile for Zverev.

His ability to play the percentages and churn out wins in third gear has been a key component in his rise to one of the all-time greats and that was all he needed on Wednesday.

It was the 31-year-old Serb's second comfortable win in the Gustavo Kuerten group and combined with Marin Cilic's 6-7, 6-3, 6-4 defeat of American John Isner later in the day, he sealed his place in the semi-finals at the year-ending tournament for the eighth time.

Djokovic appeared to be suffering with a cold and grabbed a tissue from his pocket on occasions, but after some fierce resistance in the opening set he ran away with the match.

"A win is a win," the world No 1, bidding for a record-equalling sixth title at the ATP's blue-riband event, told reporters.

"Somehow I managed to gather the strength when I needed it. It wasn't a great quality tennis match to be honest."

Zverev blazed 15 winners in the first set and for a while his baseline firepower kept Djokovic under pressure, especially at 4-4 in the first set when he had a couple of break points.

Djokovic saved the first with heavy serve but then double-faulted, offering Zverev another glimpse.

This time Zverev traded blows from the baseline before drawing Djokovic to the net but he just missed the target with a topspin lob that landed in the tramlines.

Zverev was never the same after that.

He double-faulted to concede the set and despite hanging on to level at 1-1 in the second, his resistance crumbled quickly as Djokovic reeled off the last five games.

What had promised to be a real battle petered out into a another one-sided affair and the seventh consecutive group match this year not to go the distance.

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"I guess winning the first set really counts this year. Then, I guess, you have mentally quite an edge," Djokovic, who has won 33 of his last 35 matches, said.

The scrap is now on for second place with Zverev targeting a win over Isner on Friday to reach the semi-finals for the first time after falling in the group phase last year.

He, Cilic and Isner can all still join Djokovic in the semis and calculators might be needed on Friday.

"I think I'll take it as a normal quarter-final match at a normal tournament," said the German, who beat Croatia's Cilic in his opening match on Monday.

Cilic kept his hopes of a semi-final berth alive as he came through this year's first three-setter -- improving his record at the tournament to a still dismal 2-9.

When he lost the opener to the American on a tiebreak it looked like more misery for Cilic, but he responded in superb fashion, even if he needed some help from Isner's mighty serve.

Debutant Isner double-faulted to hand Cilic a decisive break in the eighth game of the second set and then did the same to gift Cilic a break at the start of the third.

Cilic wobbled briefly, handing the break back with a woeful game, but struck again with some bludgeoning groundstrokes and kept his composure to claim the win.

"There was a little bit of luck involved," Cilic, who could have to beat Djokovic for only the third time in 19 meetings on Friday to progress, said. (Reporting by Martyn Herman Editing by Christian Radnedge and Pritha Sarkar)