The 38-year-old swatted aside Steve Johnson 6-3, 6-2, 6-2 to race into the second round, kickstarting his bid for a 21st Grand Slam title in immaculate style.
David Goffin claims an upset 6-4 7-6 (3) victory over world number one Rafael Nadal in Sydney to keep Belgium alive in their quarter-final clash at the inaugural ATP Cup team event.
Svetlana Kuznetsova pulled out of Auckland Classic due to a viral illness, while Jelena Ostapenko withdrew for "personal reasons."
The top-ranked Nadal won all of his eight matches at the revamped competition that debuted a World Cup-style format.
The underwhelming performance of the reigning champions will most likely require them to beat Rafael Nadal's Spain to stand a chance of reaching the knock out phase.
The 32-year-old Serbian tennis star avenged his loss to the 21-year-old Greek 3 weeks earlier in the Shanghai Masters, with a 58-minute victory.
Meanwhile, world number one Novak Djokovic defeated Britain’s Kyle Edmund with 7-6(7) 6-1 to reach the Paris Masters quarter finals.
Naomi Osaka has withdrawn from the WTA Finals because of a right shoulder injury ahead of her scheduled round-robin match against top-ranked Ash Barty.
US Open runner-up Medvedev, chasing his fourth ATP Tour title in a breakthrough 2019 season, will meet Germany's Alexander Zverev in Sunday's final.
The three-time Grand Slam winner says he no longer has pain in his hip and it is now a matter of building up fitness so he can play several matches in a row at tournaments.
Williams will face Canadian Bianca Andreescu for the US Open title as the American sets herself up for another attempt to clinch a record-equaling 24th Grand Slam singles title.
Champion Novak Djokovic elected to retire in the third set due to a left shoulder injury. He was facing off Stan Wawrinka who was leading 6-4, 7-5, 2-1.
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