Anna Green endured a nightmare at left fullback, caught out of position for three of the five goals, and the New Zealand side only carved out one goal scoring opportunity across the 90 minutes.
The result matched the scoreline of the team's previous encounter in September 2017, which was the last match in charge for former coach Tony Readings.
However Sermanni was quick to put the performance in context, saying the Ferns don't want to hit their peak for another three weeks.
"It wasn't an unexpected result," said Sermanni. "It was almost like we are starting pre-season and having a game within the first couple of days, against a US team that have all been playing professionally in the NWSL, been in camp for the last few days and have entered the prime of their preparation for the World Cup.
Losing 5-0 is never a pleasant occurrence [but] for parts of the game I was really pleased with how we did and the first 35 minutes we were really solid."
Sermanni characterised it as a learning experience, which will prove invaluable in the context of the wider campaign.
"We got a clear idea of where we are at, and a clear idea of what we need to do," said Sermanni. "The first two goals were more damaging than the ones late in the game. We were tiring significantly and they brought on some world class players.
"[It's] part of a preparation. We are getting prepared for June 11th. This game, and the game against Mexico, are games on that journey, not games that we are actually preparing for specifically."
The United States took the lead in the 35th minute after captain Megan Rapinoe's cross found Tobin Heath, who had a simple finish. Five minutes later Rose Lavelle volleyed home at the back post, through Green was again caught ball watching.
Veteran Carli Lloyd scored two second half goals after coming off the bench, before Sam Mewis smashed home from long range.
Ferns midfielder Rosie White, who marked a century of caps, came closest for New Zealand with a first half effort over the bar.
The Ferns face Mexico on May 22 in New York in their next pre-World Cup match.