Tone-Fitzpatrick, who is studying law and arts at Auckland University, said she was shocked when she got a phone call from Silver Ferns' coach Ruth Aitken on Monday night to tell her the good news.
"I'm still just trying to process it really," said Tone-Fitzpatrick.
"I wasn't really thinking about [test selection] at all. I just wanted to get through the camp and hopefully learn what I have to do to get to the next level."
With Williams and Grant missing through injury, Aitken decided to inject some youth in the defensive end.
But she admits while she has been impressed with Tone-Fitzpatrick and Cullen, they have a lot to learn about the elite environment.
"They both just bring that lovely young exuberance and willingness to have a go.
"They're both very athletic and have a lot of natural talent. There's a lot of learning to be had but there's nothing like being in the environment to get a handle on it," she said.
While Aitken will gauge the progress of the two teenage defenders over the next few weeks before deciding whether they will get court-time in the upcoming series against England, Latu is certain to make her long-anticipated debut for New Zealand after a false-start earlier this year.
Latu was originally named in the Ferns' world championship side pending an appeal to the International Court of Arbitration for Sport over the star shooter's eligibility. The appeal failed and Latu was scratched from the team for Singapore. But now having served out a four-year international stand-down, Latu can officially represent her country of birth.
The 24-year-old shooter said after having watched from the sidelines for so long, she felt more relief than excitement to finally be eligible to wear the black dress.
Midcourter Laura Langman will captain the side for the five remaining tests this year in the absence of Williams, who is recovering from ankle surgery, with Irene van Dyk as vice-captain.
NZ squad
Silver Ferns to face England:
Laura Langman (c), Kayla Cullen, Leana de Bruin, Paula Griffin, Joline Henry, Cathrine Latu, Liana Leota, Anna Scarlett, Anna Thompson, Sulu Tone-Fitzpatrick, Maria Tutaia, Irene van Dyk.
New Caps
Cathrine Latu
Born: October 25, 1986, Auckland
Height: 1.89m
Positions: GS
ANZ franchise: Northern Mystics
Previous test caps: 30 (Samoa)
Kayla Cullen
Born: February 13, 1992, Auckland
Height:1.85m
Positions: GD, WD
ANZ franchise: Northern Mystics
Sulu Tone-Fitzpatrick
Born: August 20, 1992, Auckland
Height: 1.88m
Positions: GK, GD
ANZ franchise: Waikato/BoP Magic.