The Swiss, playing near-flawless tennis, broke once in the first set and twice in the second to beat his rival for the first time since 2015
Roger Federer produced a serving masterclass as he avenged his Wimbledon defeat by Novak Djokovic and qualified for the last four of the ATP Finals Thursday with a 6-4, 6-3 victory.
The Swiss, playing near-flawless tennis, broke once in the first set and twice in the second to beat his rival for the first time since 2015.
Roared on by a raucous packed house at London's O2 Arena, the six-time champion dropped just seven points on his serve in the first set.
Defeat spells the end of Djokovic's bid to overtake Rafael Nadal and finish as year-end No 1.
The third-seeded Federer, making his 17th appearance at the ATP Finals, reached his 16th semi-final.
Djokovic needed to win the title to have a chance at knocking Nadal off the top spot, but now the Spaniard is guaranteed to finish the year as the top-ranked player for the fifth time, tying him with Federer, Djokovic and American Jimmy Connors.
Big 3️⃣ math:— US Open Tennis (@usopen) November 14, 2019
Federer def. Djokovic = Nadal will finish the year at #☝️ pic.twitter.com/4nxzB2OoGl
Federer finishes second in Group Bjorn Borg, behind Dominic Thiem, who beat both Federer and Djokovic earlier this week.
The Swiss will face the Group Andre Agassi winner Saturday.
Djokovic looked nervy at the start of the winner-takes-all contest, double-faulting twice in the third game, in which he was broken to love.
As cries of "Let's go Roger, let's go" rang around the cavernous stadium, Federer was dead-eyed on his serve, hitting eight aces, including a second-serve ace, in the first set.
Federer's service level dipped in the second set and Djokovic earned his first break point of the match in the fourth game, which the Swiss saved.
He broke Djokovic again in the fifth game to take an iron grip on the match and repeated the trick to take the second set 6-3.
It was the 49th time Federer and Djokovic had met.
Djokovic, who now holds a 26-23 advantage, had won their past five meetings, including their epic five-set battle in the final at Wimbledon in July, during which he saved two championship points.
In Thursday's early match in Group Bjorn Borg, which was a dead rubber, eighth seed Matteo Berrettini beat Thiem 7-6 (7/3), 6-3.
In doing so, the 23-year-old became the first Italian to win a match at the season-ending championship.
Berrettini arrived in London at a career-high No 8 in the ATP ranking after starting the season outside of the top 50.
Fifth seed Thiem did not hit the heights he reached during his three-set win against Djokovic, notching just 12 winners compared with 50 against the Serbian.
Stefanos Tsitsipas has already qualified for the semi-finals from Group Andre Agassi, leaving Nadal, defending champion Alexander Zverev and Daniil Medvedev to scrap it out for the other spot Friday.