Roger Federer produced a devastating display to down Novak Djokovic in straight sets and reach the semi-finals at the ATP Finals in London. In their first meeting since their Wimbledon final epic in July, Federer was ruthless on serve as he came through 6-4 6-3 to earn a first win over the Serb since 2015. Respective defeats to Dominic Thiem added extra spice to the occasion, with both players knowing only victory would send them into the last four alongside the Austrian. And it was Federer who made the stronger start, cracking his opponent's serve in the third game and remaining flawless on his own to take the first set in 36 minutes. When Djokovic did belatedly muster a break point in the fourth game of the second set, it only spurred Federer to new heights. Not only was it duly swatted away, but the 38-year-old then broke immediately to leave him on the cusp of the semi-finals. Djokovic's resistance was over as Federer soon broke again to love to seal victory, partial atonement for that fifth set tie-break agony at Wimbledon. Federer, who served for the title in that marathon match at SW19 in the summer, was immediately asked what he did differently. "I won the match point," he quipped. Earlier, Matteo Berrettini signed off his debut ATP Finals with a consolation victory over Thiem. The Italian was already eliminated after earlier defeats to Djokovic and Federer but roused himself to beat the Austrian 7-6(3) 6-3 in London. Thiem was already assured of a spot in the semi-finals as group winner. "It's been an unbelievable season," said Berrettini. "I didn't expect to be here but I hope to be here next season. I'm not feeling really great physically but I'm happy." It was Berrettini's third victory over a top-five player in 2019, while the 23-year-old becomes the first Italian to win a match at the season-ending finals. He also reached the US Open semi-finals and won two ATP titles in his breakthrough season. Thiem and Federer's opponents in the last four will become clear on Friday when Group Andre Agassi concludes.