London - Alexander Zverev kept his nerve at the key moments to beat 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 contest littered with unforced errors at London's O2 Arena, the 21-year-old third seed recovered from a break down in each set to establish an early foothold 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 and had two break points to establish a 4-0 lead.
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 and, buoyed by his comeback, Zverev edge the tie-break.
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, dug deep to save match point when trailing 4-5 but the set went to another tie-break, which Zverev dominated, winning 7/1 to seal the match.
There were a total of 78 unforced errors in the contest -- with Cilic hitting 46 of those -- as both players struggled to adapt to the court conditions.
"The court is difficult," said Zverev. "The court is very fast. And it's very high-bouncing as well. So, it's more difficult than in other tournaments, so everybody has to kind of find their rhythm in the first match.
"But at the end of the day it was a pretty good match. I'm happy to get the win. That's the most important thing. It's important -- this tournament is big. It is very important to all of us players."
Zverev has long been talked about as one of the brightest talents of his generation and will be keen to make a statement in London after again flattering to deceive at Grand Slam level this season.
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.