Federer beaten by brilliant Thiem on first night of ATP Finals
Roger Federer's campaign to win a third ATP World Tour Finals in London got off to the worst possible start as he was beaten 7-5 7-5 by Dominic Thiem.
Federer can still qualify for the semi-finals - he lost his first match last year to Kei Nishikori - but having been beaten in straight sets, he will have to win his two remaining round-robin matches against Novak Djokovic and Matteo Berrettini.
Thiem meanwhile has never been beyond the group stages despite having qualified for the year-end championship in each of the last three seasons.
"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."
Thiem is one of the few active players with a winning record against Federer, having only lost twice to the Swiss in their six meetings, even beating him on grass as well as his favoured clay.
And he proved a worthy opponent right from the off, winning the first three points of the match on Federer's serve before converting his third break point when he produced an unforced error on his forehand.
The Austrian's defence would become a key theme as the match was played, in the most part, off Federer's racket and while the 20-time Grand Slam winner did break back as errors crept into the Thiem serve in the fourth game, he would drop his serve again at the crucial moment.
Serving at 5-5, another loose Federer forehand handed Thiem break point and this time he attacked the backhand wing, rewarded with a chance to serve for the set that he would not turn down.
Federer bit down in the second set, working Thiem over on serve and responding with speedy service games of his own to heap the pressure back on him.
But Thiem, powered by the knowledge of those previous victories over the legendary player, held the baseline and would not relent.
Federer should have known something was amiss when he smash an overhead into the net at 3-3 in the second - he is perhaps the most reliable in the world when the ball is sky-high.
He held his serve and did so again despite being pushed to deuce two games later but the relentless Thiem pressured paid, once again, at 5-5.
Federer missed a volley he would have expected to make to give Thiem 0-40, and at the first time of asking a fine return completed the break of serve.
Never underestimate the difficulty of the finishing line though and Federer made him sweat, twice creating chances to break and saving a match point before Thiem finally closed it out it one hour, 41 minutes to join Novak Djokovic atop the 'Group of Death'.