Pegula failed to advance into the semis in her first 23 main draw appearances at a major before reaching the final in New York a year ago, when she lost to Aryna Sabalenka.
She has since been eliminated in the fourth round at the Australian Open, the third round at the French Open and the first round at Wimbledon.
Being back on hard courts at the US Open has been a recipe for success. She has not yet lost a set.
"I think I've been playing some really good tennis," Pegula said. "I've just been playing very solid. I've been having very good quick starts, so I really wanted to do that today, especially against someone like (Krejcikova) who's very dangerous."
Sabalenka, now the top-ranked women's tennis player in the world, could be Pegula's next opponent if she beats Marketa Vondrousova in their match on Tuesday night. That's also at Arthur Ashe Stadium, where Pegula made quick work of Krejcikova.
"It's crazy to look now and think that I'm really comfortable coming out here playing on big courts in big matches on the best court in the world with the craziest crowd against the best players," Pegula said. "It's pretty crazy, and it's something 10 years ago I never thought I'd be good at this, but I guess I am."
Pegula, 31, is seeded fourth and aiming for her first Grand Slam championship.
Pegula was not perfect, but she played well enough to stay in control at all times and broke Krejcikova one final time to end the match in under 90 minutes — on her first match point attempt.
