US Open 2019 at a glance - Day one
Serena Williams began her quest for a 24th Grand Slam singles crown by crushing five-time major winner Maria Sharapova 6-1, 6-1 to reach the second round of the US Open.
Williams needed only 59 minutes to rout her Russian rival, improving to 20-2 all-time against Sharapova with her 19th consecutive victory in the rivalry.
"Whenever I play her I bring out some of my best tennis," Williams said. "She's such a good player. When you play her you have to be super focused."
Novak Djokovic began his US Open title defence in style with a 6-4, 6-1, 6-4 win over Spaniard Roberto Carballes Baena in the first-round action.
The top-seeded Serbian, who has claimed four of the last five Grand Slam titles, took some time to settle in to his first career meeting with Carballes Baena but after a tight first set was off to the races.
Djokovic got an early break to go-ahead 2-0 in the second set and never looked back as the three-times champion used a mix of deft volleys, solid movement and great instincts to remain undefeated in 14 career first-round matches in New York.
Roger Federer gave India's Sumit Nagal a harsh introduction to Grand Slam tennis as the five-time champion weathered an early storm for a 4-6 6-1 6-2 6-4 win to reach the second round of the US Open.
When the match began it was Federer who surprisingly looked intimidated under the Arthur Ashe Stadium lights as he sprayed errors all around the court but the Swiss great woke from his slumber in time to turn the match resoundingly in his favour.
French Open champion Ashleigh Barty survived a first-round scare at the US Open, staving off Kazakhstan's Zarina Diyas 1-6, 6-3 6-2 after struggling to find her footing as the action kicked off at Arthur Ashe Stadium.
The 25-year-old Diyas took five straight games in the first set, as the world No.2 committed a string of unforced errors and got in just 25 per cent of her first serves.
Barty was able to stop the pain midway through the second set, converting on a crucial break-point opportunity to take a 5-3 lead and reverse the momentum in what had been a one-sided match.
Third seed Karolina Pliskova struggled but advanced.
Pliskova, a 2016 US Open runner-up, edged 138th-ranked Czech compatriot Tereza Martincova 7-6 (8/6), 7-6 (7/3) after exchanging service breaks over the first six games.
"Not my best performance," Pliskova said. "Not happy that I lost three breaks of serve in a row."
One of eight former US Open champions in action on day one, Angelique Kerber crashed out prematurely once more following early exits at Roland Garros and Wimbledon.
The German 14th seed, who won the title in 2016, was beaten in three sets by France's Kristina Mladenovic.