Serena Williams won her first US Open two years before Caty McNally was born - and she had to use all her experience to avoid crashing out to the 17-year-old.
Williams, the 23-time grand slam winner and last year's runner-up, found herself staring down the barrel of a major upset against her fellow American.
McNally, standing up to Williams' second serve, had the 37-year-old on the ropes when she took the first set.
But Williams dug deep in a dramatic late-night session in New York to win 5-7 6-3 6-1 and progress to the third round.
"I think she really came out and played well, she showed no fear, she had nothing to lose and she played like it," said a relieved Williams.
"It's refreshing, and it's a different game. I don't get to play players like that too often.
"I knew I could get better, I tried to let Serena come through a little bit. In the first set she was picking out my wide serve, but I have two other serves so I tried them.
"I survived tonight. I'm not too pleased with the way I played at all. Serena you made way too many errors. But I'm still alive and I'll do better, I promise."
Venus Williams saved five match points before being beaten by Elina Svitolina
Serena's sister Venus Williams saved five match points before her 21st US Open campaign was ended by Elina Svitolina.
Serving to stay in the match, the 39-year-old showed her never-say-die spirit remains undimmed in a marathon penultimate game.
She forced Svitolina to serve for the match, the Ukrainian fifth seed eventually crossing the line for a 6-4 6-4 victory.
"It was a really incredible match," said Svitolina.
"It was really special. It's unbelievable what Venus does, on and off the court she is a big inspiration.
"I had to stay very focused and try to put an extra ball over the net. She knows how to handle moments like this, it was incredibly tough and only a few points decided the match."
Elsewhere...
British number one Johanna Konta made a brief appearance on court for her match against Margarita Gasparyan, but the rain started again and prevented the players from even warming up.
Shortly afterwards, the USTA announced that play for the day on the outside courts had been cancelled.
Cori Gauff, the 15-year-old who lit up Wimbledon on her grand slam debut, has won her first match at the US Open.
Gauff dropped the first set to Russia's Anastasia Potapova but hit back to win 3-6 6-2 6-4.
Third seed Karolina Pliskova is also through to the third round, the 27-year-old from the Czech Republic beating Georgia's Mariam Bolkvadze 6-1 6-4.
Second seed Ashleigh Barty beat Lauren Davis of America 6-2 7-6 (2) and 10th seed Madison Keys saw off China's Zhu Lin 6-4 6-1.
Sloane Stephens, the champion just two years ago, crashed out to Russian youngster Anna Kalinskaya.
Kalinskaya, 20, won her first ever grand slam match 6-3 6-4 against the below-par American.
