ATP Finals: Roger Federer stunned by Dominic Thiem in opening group match

20-time Grand Slam champion Roger Federer suffered a major blow as he slumped to a shocking straight sets defeat at the hands of Dominic Thiem of Austria in the opening round robin clash of the Association for Tennis Professionals (ATP) Finals at the O2 Arena in London on Monday. 

Swiss tennis maestro Federer, who is bidding for his seventh ATP Finals title and first since 2011, struggled against Thiem's aggressive game to eventually suffer a 5-7, 5-7 defeat in a thrilling clash that lasted one hour and 40 minutes. 

Following the clash, Thiem said playing against Federer has always been an honour for him. 

“To beat him, everything has to fit together. Today, I think was the case again. I played very well. I played a good match in general. Serving, returning well, which is probably the two most important things indoors," the ATP quoted Federer as saying. 

“In general, it's a very nice victory for me. I mean, every time I play against him is a big honour. Beating him is even better, and especially on this surface indoors, where he's feeling well usually," he added. 

In the opening set, Federer made a worst possible start as he was broken in the first game before he bounced back to level the score. However, Thiem broke yet again in the 11th game before serving out to clinch the set. 

Though Federer forced two break points in the second set to stay alive in the clash, Thiem held his nerve to win on his second match point and eventually emerge victorious.

Notably, Thiem-- who is playing at the season-final tournament for the fourth consecutive year--has clinched his first opening group match win with victory over Federer. 

With the win, fifth-seed Thiem has also improved his ATP Head-to-Head record against Federer to 3-4.

Federer will now lock horns with Matteo Berrettini of Italy while Thiem will face Serbian tennis star Novak Djokovic in next group clash.