By Martyn Herman
LONDON (Reuters) - World number one Novak Djokovic produced a rock-solid performance to dismantle the challenge of German firebrand Alexander Zverev at the ATP Finals on Wednesday.
The Serb, 10 years older than his opponent, was made to work hard in an absorbing first set but Zverev's challenge crumbled at the O2 Arena as Djokovic won 6-4 6-1 to move within sight of the semi-finals.
Five-times champion Djokovic, who will end the year as world number one for the fifth time in his career after a storming second half of the season, leads the Gustavo Kuerten group with two wins and is yet to drop a set.
He will advance into the last four of the season-ending tournament for the eighth time if John Isner beats Marin Cilic in the evening session.
"I don't think it was breathtaking tennis from both of us but a win is a win," Djokovic, who hit only three winners in the first set compared to Zverev's 15, said on court.
"I started to play better in the second set and he was making a lot of unforced errors, which helped me win."
All seven round-robin matches played so far this week have been won in straight sets, but for a while it appeared as though the crowd would be given full value for their ticket.
The 21-year-old Zverev, who has a Tour-leading 55 match wins this year, went toe-to-toe with Djokovic from the baseline and looked to have the firepower to worry his opponent.
Djokovic netted a routine forehand when serving at 4-4 30-30, offering Zverev a break point that he did not convert.
Djokovic then double-faulted to give Zverev another chance to serve for the set but this time the German went just wide with an attempted lob after the Serb ventured into the net.
The net game always looked dangerous for Zverev and so it proved as he double-faulted to concede the set.
Zverev looked deflated and despite holding serve for 1-1 he offered no more resistance as Djokovic won five games in a row to seal his 33rd win in his last 35 matches.
His final group match is against Cilic while Zverev will hope to bounce back against Isner.
(Reporting by Martyn Herman; Editing by Christian Radnedge)