But it was all smiles from defending men's champion Grigor Dimitrov, who powered into the quarter-finals in straight sets.
In the opening match on Pat Rafter Arena, Birrell matched world number eight Navarro stroke-for-stroke to put a dent in the American's Australian Open preparations.
Navarro had a stellar 2024, going deep in all four Grand Slams including a run to the semi-finals of the US Open.
She was expected to brush aside the challenge of 26-year-old Birrell, who has never broken into the top 100 in her 10 years on the WTA Tour.
But the Australian, who trains at the Queensland Tennis Centre and is ranked 113, was not intimidated and delighted her home crowd with a nerveless display.
Birrell, who will face 35th-ranked Russian Anastasia Potapova in the last 16, said she embraced the role of underdog.
"I guess it's kind of like one of those situations where you have no choice, you just have to go for it," she said.
"That was kind of the attitude I had going into today -- I think that really benefited me, along with having played a lot of hours on this court here."
Dimitrov continued his love affair with Brisbane by easing past another Australian wildcard, Aleksandar Vukic, 6-2, 7-6 (7/5).
The Bulgarian world number 10 raced through the first set then held off Vukic as the Australian raised his game in the second.
"I'm starting to find my game but I think there is still a lot to do," said Dimitrov, who has won the tournament twice and finished runner-up once.
"So hopefully with each match I can build up a little bit more and play better and better."
He will play either Australian Jordan Thompson or American Alex Michelson for a place in the last four.
Djokovic and Kyrgios teamed up for a rare doubles appearance this week, but after winning a close first round were stopped by top seeds Michael Venus of New Zealand and Croatia's Nikola Mektic 6-2, 3-6, 10/8.
Venus and Mektic came from three points down to clinch the match tiebreak.
"Playing those guys, they hit some unbelievable shots," Venus said.
"Down three points in the tiebreak, the way they were serving, it wasn't looking great but somehow we got back in there."
