Novak Djokovic has made the ideal start in his quest to win a record-equalling sixth ATP Finals title.
The world number two easily beat Italy's Matteo Berrettini in straight sets at the O2 Arena in London.
However, things weren't as rosy for Roger Federer, who was beaten by Dominic Thiem in his tournament opener.
It's the second year in a row that the seven-time Dubai Duty Free Tennis Championships winner has lost his opening match at the ATP Finals.
US keep Super 8 hopes alive with 31-run win over Namibia in T20 World Cup
The United States kept their hopes of reaching the Twenty20 World Cup's Super 8 stage alive as they ended their Group A campaign with two straight wins, after beating Namibia by 31 runs on Sunday.
Sweden's Ponsiluoma shows nerves of steel to win men's pursuit gold
Martin Ponsiluoma held his nerve and hit his final five shots to win the men's 12.5km pursuit Olympic race in spectacular fashion on Sunday to finally secure a biathlon medal for Sweden at the Winter Games.
Holder, Hope help West Indies thump Nepal, make T20 World Cup Super Eight
West Indies charged into the Super Eight Stage of the Twenty20 World Cup after the two-times champions defeated Nepal by nine wickets in Mumbai on Sunday, with Jason Holder's four-wicket haul and Shai Hope's half-century setting up the win.
Banton blitz lifts England past Scotland in T20 World Cup
England beat Scotland by five wickets in the Twenty20 World Cup on Saturday to move up to second in Group C thanks to Tom Banton's unbeaten half-century at Eden Gardens.
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