By Martyn Herman
LONDON (Reuters) - Roger Federer's pursuit of a seventh ATP Finals title got off to a poor start on Sunday when the Swiss great lost 7-5 7-5 to Austrian Dominic Thiem in his opening group match.
From the moment Federer dropped his opening service game he never seemed entirely comfortable and Thiem took full advantage to claim his third win over Federer this season.
Federer recovered the early break but a couple of lazy strikes at 5-5 cost him his serve again before Thiem closed out the opener in confident fashion.
Thiem, beaten by Federer in the group stage last year, upped the power in the second set to keep Federer under pressure and unable to dictate and pounced again to break at 5-5.
Federer saved one match point in the following game and had a break point but there was no escape and Thiem pummeled away a forehand to claim victory and join Novak Djokovic at the top of the group after the Serb earlier beat Matteo Berrettini.
Thiem, who now leads Federer 5-2 on head-to-head, will face Djokovic on Tuesday while Federer will be up against Italian debutant Berrettini.
"It was a great performance and special to get my first win in an opening group match in the Finals," Thiem said.
"It's overwhelming to enter this arena and we had a good and close match. It's always special to beat Roger as he is a legend."
Federer also lost his opening group match last year to Kei Nishikori but recovered to reach the semi-finals.
(Reporting by Martyn Herman; Editing by Toby Davis)