It was also the third straight loss this year against Thiem for Federer who jumped off the Paris Masters tournament last week to be fresh in London.

- To meet him is a great honor, to beat him is even better, says Thiem.

The result means Federer will probably have to defeat both Novak Djokovic and Matteo Berrettini (Djokovic won that meeting by 6-2.6-1) to advance to the semi-finals of the Björn Borg group.

- It will be like a normal tournament from this match. I can't afford any more loss. It's just like it's been every week for the last 20 years. From that point of view, this is nothing new, says 38-year-old Federer.

In the Andre Agassi group, world set Rafael Nadal will go in against Alexander Zverev tonight. The Spaniard had a slight burst in a stomach muscle last week but will start.