Alexander Zverev saw off the challenge of Marin Cilic 7-6 (7/5), 7-6 (7/1) at the ATP Finals on Monday as Novak Djokovic prepared to launch his campaign for a record-equalling sixth title.

In a match littered with dozens of unforced errors at London’s O2 Arena, the 21-year-old Zverev recovered from a break down in each set to set the early pace in the Guga Kuerten group.

Australian Open finalist Cilic came into the match with the dispiriting record of five consecutive defeats against the young German but broke Zverev at the first opportunity.

The 30-year-old Croatian, who has a reputation for faltering when under intense pressure, kept his nose in front but was broken as he served for the set at 5-3.

The set went to a tie-break and, infused with fresh belief, third-seed Zverev established a foothold, taking it 7/5.

In the second set, Cilic again drew first blood, breaking Zverev in the seventh game to establish a 4-3 lead but the German responded immediately to level at 4-4.

Cilic, seeded fifth in London, did well to save a match point when trailing 4-5 but the set went to another tie-break, which Zverev dominated, winning 7/1.

World number one Djokovic, who has won the event five times, later plays debutant John Isner – at 2.08 metres (six feet 10 inches) – the tallest player ever to compete at the year-end championships.

The Serb, guaranteed to end the year in top spot, has enjoyed a remarkable second half of the year, winning Wimbledon and the US Open as he surged up the rankings.

Roger Federer lost to Kei Nishikori in straight sets in the Lleyton Hewitt group on Sunday, leaving him in grave danger of failing to qualify for the semi-finals for only the second time in his career.

An early exit for the Swiss veteran would deprive the tournament of star power, with world number two Rafael Nadal already missing through injury.

The title is contested by the eight players who have accumulated the most ranking points over the season and is in a round-robin format, with the best four players reaching the knockout semi-finals stage.