Britain's top players Tim Henman and Greg Rusedski are out of the first-round tie in Sydney, starting on February 7 due to injury.
But Fitzgerald refused to underestimate the hobbled British team.
"We are treating the opposition like they are at full strength in our preparation," Fitzgerald said. "They (Britain) dislike losing like we do and that's enough stimulation to get the best out of ourselves."
Philippoussis, the hero of Australia's victory over France in the 1999 final, has not played Davis Cup in recent years due to injuries and disputes with previous coaches.
"I've always said if I'm fit I'm going to play and nothing has ever changed," Philippoussis said. "I don't think I've exactly been fit the last three years, so I'm fit (now) and I am here."
Fitzgerald said Hewitt had needed time to get over his fourth-round loss at the Australian Open to Moroccan Younes El Aynaoui.
"He's done a fair bit of work this week," Fitzgerald said.
Hewitt and Philippoussis are likely to fill the two singles spots with Todd Woodbridge and Wayne Arthurs to play for Australia in the doubles.
In the absence of Henman and Rusedski, Britain have named Arvind Parmar, Miles Maclagan, Alex Bogdanovic and Alan Mackin in the team.