Fresh off a run to the Wimbledon boys final, Hewitt beat former world No.37 Marcus Giron - currently ranked 85 - 6-3 6-4 in Washington DC on Tuesday (Wednesday AEST).
The Australian, who didn't face a break point, became the fourth man born in 2008 or later to win a tour-level match and youngest Washington match winner since Japan's Kei Nishikori in 2007.
The teen qualified thanks to walkovers after leading both Dusan Lajovic and top-seed Zach Svajda and has surged 255 places to No.357 in the ATP's live rankings.
He'll play the winner of countryman and mentor Alex de Minaur and former world No.3 Stefanos Tsitsipas in the next round, with their later match washed out.
He had warmed up with father Lleyton, the former world No.1, two-time major winner and 2004 Washington champion, who told him to lap up the moment.
"(He told me) mainly to enjoy it, because it was my first tour level match ... and to take every point one-by-one, to enjoy the atmosphere and to enjoy the crowd and that is what I did," he said.
Cruz said he was handling the pressure of carrying the Hewitt name "as best as I can".
"There is obviously a little bit of pressure, but I try to take the positives out of it as well and I am happy with how I am dealing with it so far," he added.
"I am really happy with how I competed today. I came out and started pretty well and I am thrilled to get through to the next round."
De Minaur is defending his title in the American capital and fresh off a two-day Portofino honeymoon after marrying fellow pro Katie Boulter in Tuscany last month.
"It was an amazing time, everything we could have ever imagined," De Minaur, coming off the disappointment of an upset fourth-round Wimbledon loss, said.
"It was like our own little movie. We were able to celebrate with family and friends and have a very small honeymoon.
"Then we're on a flight here to Washington and getting our heads screwed back on and back to being tennis players."
Hewitt said it would be a "dream" to square off against de Minaur, who he likened to a big brother.
"Obviously if I play Alex, that would be a dream to share the court with him," the teen said.
"He's like my older brother. I've known him basically my whole life. He's stayed with us and we have Christmas together, also. He's taught me so much, I think that would be a pretty cool memory for us."
5°C