Serbia's Novak Djokovic serves against Italy's Matteo Berrettini during their men's singles round-robin match.
LONDON - Roger Federer faces an uphill task to qualify for the last four of the ATP Finals after defeat by Dominic Thiem on Sunday following a masterclass by Novak Djokovic.
Six-time champion Federer was beaten 7-5, 7-5 in the evening match on the opening day after Djokovic had swatted aside debutant Matteo Berrettini in the earlier Group Bjorn Borg contest in London.
Federer and Djokovic will meet later in the week in a repeat of this year's epic Wimbledon final, with two players advancing from the four-man group.
Djokovic began his quest for a record-equalling sixth ATP Finals title and sixth year-end number one spot a 6-2, 6-1 romp against Berrettini.
The Serbian was last week nudged off the top position in the rankings by Rafael Nadal, who is also in the eight-man field in London.
Djokovic must reach the final at the O2 Arena to stand any chance of dislodging Nadal but there are major questions over the Spaniard's fitness.
If the Serbian does claim the year-end number one spot, he would match the record of Pete Sampras and victory at the end-of-season event would equal Federer's mark.