Novak Djokovic, the No 1-ranked man in the world, was at his most ruthless against Irish qualifier Conor Niland, who threw in the towel after falling 6-0 5-1 behind.
Even before the match began, it did not seem like a fair fight and so it proved. The Irishman was suffering from food poisoning and looked as green as the balls he was trying to retrieve at Arthur Ashe Stadium.
He lasted just 44 minutes before he quit, given no respite from Djokovic on a hot and sunny day at Flushing Meadows.
Djokovic showed no ill effects of the shoulder injury that forced him to quit last week's Cincinnati final against Andy Murray but he would have been happy to get off the court early.
He has lost just two matches this year, claiming the Wimbledon and Australian Open titles along the way, but he remains driven by greater heights.
The signs are already ominous for his main opponents that it will take a great performance to beat him for the title.
Roger Federer won his opening match on Monday in straight sets, while Rafa Nadal and Andy Murray were yet to open their campaigns.
Caroline Wozniacki, the women's world No 1, provided a glimpse of the ruthless streak her critics have accused her of lacking, demolishing unseeded Spaniard Nuria Llagostera Vives 6-3 6-1.
The Dane, who has been in the headlines as much for her relationship with golfer Rory McIlroy as her performances on court, smashed 22 winners in a lopsided victory that took just 80 minutes.
It was as impressive a start to the tournament as any women has made so far but Wozniacki was still forced to defend herself.
A finalist in New York two years ago, she is still chasing her first grand slam title, giving her doubters all the ammunition they need.
"They can say what they want," she snarled. "I've won a lot of tournaments. I'm number one in the world."
"There are a lot of things to my game I can still improve, but everyone can. I'm on the right track."
