Hewitt beat France's Arnaud Clement 6-7, 6-4, 7-6 to reach the third round of the Hamburg Masters on Wednesday after telling an umpire to return to the minor tour.
"You go back to the satellites, mate. You go back to the satellites and learn the rules where you belong," the 22-year-old told the umpire.
"I think he's just getting his competitive juices flowing for the French Open, just trying to get up," Fitzgerald told reporters.
The Hewitt footage was replayed on Australia television on Thursday alongside this week's clash between cricketer Glenn McGrath and West Indies batsman Ramnaresh Sarwan during the fourth Test in Antigua.
The much taller McGrath angrily pointed his finger at the batsman and swore several times. Australia's top cricket official James Sutherland said on Tuesday it was an "ugly incident" which reflected poorly on the sports-loving nation and cricket's image worldwide.
Fitzgerald, however, was backing Hewitt's tough stance.
"He's just competing and, you know what, you have to be yourself, you have to compete," said Fitzgerald, who will be relying heavily on Hewitt in September's Davis Cup semi-final against Switzerland.
"He admits he walks over the white line. There's a little bit of fever there but that's what makes him such a great player."
Hewitt has never been far from controversy. He once described spectators in his home town of Adelaide as "stupid" after they cheered an opponent in 2000, and he was fined at the 2001 French Open for calling the chair umpire a "spastic".
He was also at the centre of a race row at the 2001 U.S. Open over a comment he made to a court official during a match against American James Blake.