Dominic Thiem continued his dominance of Roger Federer this year with a straight-sets victory on the opening day of the ATP Finals.

Thiem had beaten the 20-time grand slam champion twice this season and completed the hat-trick with a 7-5 7-5 victory at the O2 Arena on Sunday.

The fifth-seeded Thiem has not reached the semi-finals in his three previous appearances in London, but made a confident start after Novak Djokovic beat Matteo Berrettini in the opening Group Bjorn Borg match.

Thiem served and returned superbly, improving his record against Federer to five victories and two defeats as he bids to end a fruitful 2019 by claiming a sixth title of the year.

Federer dominated the robin-round encounter between the two in this tournament last year, but was a break down after firing a forehand into the tramlines in the opening game.

The Austrian was pegged back at 2-2 when Federer forced an error with a ferocious backhand, yet the six-time champion was unable to cut out the errors and trailed 6-5 after Thiem pounced on his second serve.

Thiem served out the set with great composure and both players had a packed crowd purring as they showed their athleticism with classy winners at the net early in the second.

Federer fizzed a majestic backhand winner down the line and saved a break point before levelling at 2-2, while Thiem continued to pepper his veteran opponent with powerful, accurate ground strokes.

Thiem drew gasps with a scorching one-handed backhand winner and broke to love to lead 6-5 when Federer netted after a blistering return was arrowed at his feet.

Federer went down with a fight, but the third seed was beaten when he dumped a backhand into the net after seeing two points to get back on serve come and go.