By Martyn Herman
LONDON (Reuters) - World number one Rafael Nadal made a disappointing start at the ATP Finals as he suffered a first career loss to Germany's reigning champion Alexander Zverev, going down 6-2 6-4 on Monday.
The Spaniard, seeking a first title at the event and hoping to secure the year-end number one ranking for a fifth time, showed no sign of the abdominal injury sustained in Paris this month, but was comprehensively outplayed.
Zverev broke serve in the fifth game and again two games later when Nadal blazed a wild forehand wide.
The 33-year-old dropped his serve again at the start of the second set and with Zverev rock solid on his own delivery he never allowed the Spaniard a glimpse of an opportunity.
The statistics told the story of the match with Zverev striking 26 winners to Nadal's 13 and winning 30 of 34 points behind his first serve.
World number seven Zverev, 22, has now beaten Roger Federer, Novak Djokovic and Nadal in consecutive matches at the O2 Arena where last year he claimed the biggest title of his career.
It proved quite the day for ending losing streaks.
Just as Zverev claimed a first win in six meetings with Nadal, Stefanos Tsitsipas had earlier beaten Daniil Medvedev 7-6(5) 6-4 in the battle of the debutants having been 0-5 against the Russian in previous meetings.
Nadal, who missed last year's ATP Finals and played only one group match in 2017 because of injury, arrived in London with a 640-point lead over Novak Djokovic in the rankings.
With 200 points for each round-robin match, Djokovic cut that deficit on Sunday by thrashing Italian Matteo Berrettini.
Nadal takes on Medvedev in his second round-robin match on Wednesday in a repeat of this year's U.S. Open final in which he took his Grand Slam haul to 19. Zverev faces Tsitsipas next.
Djokovic, seeking a record- equaling sixth ATP Finals title, can close the gap further on Tuesday when he takes on Austrian Dominic Thiem.
(Reporting by Martyn Herman; Editing by Ken Ferris)