The 19-time Grand Slam champion Roger Federer endured a first-round scare from US 19-year-old Frances Tiafoe 4-6, 6-2, 6-1, 1-6, 6-4 in a tough battle.
After withdrawing from Cincinnati, Federer went into this match with limited preparation because of his back problems.
He was however relieved that a 20th Grand Slam title is still on the cards and said: "It's important to go through all of these emotional roller coasters early. That pressure is not fun but it's what you have to go through.
"To get through a five-set match you've got to be healthy. I believe this is going to give me great confidence in my game and my body."
The 36-year-old continued, "I enjoyed myself even though I was maybe tired and nervous at the end. It was cool to be part of that match. I'm really excited I won."
A disappointed Tiafoe was not secretive about how his performance tested Federer saying, "He won by the skin of his teeth."
Meanwhile, World number one Rafael Nadal beat Dusan Lajovic 7-6 (8-6) 6-2 6-2 to reach the second round of the US Open.
The 15-time Grand Slam champion mentioned that "The first round is never easy, there are nerves out here when you play in his amazing place."
It is a possibility that Nadal and Federer, who have never faced each other at the US Open, could meet in this year's semi-finals.
Federer is also the only player who can change Nadal’s fate as world number one at the US Open, AFP adds.