She then had a number of opportunities to break Mladenovic's serve but it wasn't until the 19th game that it came after the Frenchwoman produced her 10th double fault of the match on break point.
Gavrilova then held her nerve and claimed the incredible victory after 2hr 51 minutes with her second match point.
During her post-match interview, the Aussie had a rather embarrassing slip of the tongue while trying to describe her comeback victory.
"I just yeah.. I am good from behind. Oh.." she said.
"I've got nothing in my head," she gushed afterwards. "I'm just really excited and I want to hug the whole stadium.
"I just want to play again, I want to play again on Sunday. I'm just really happy and I can't explain it."
Gavrilova had a breakout season last year, taking some big-name scalps including Maria Sharapova and Ana Ivanovic, as she rose to No.39 in the world.
The 21-year-old captured the nation's attention when she outclassed No.7 seed Petra Kvitova in the second round.
She jumped out of the blocks quickly again last night, breaking in the third game before giving the advantage straight back to the statuesque Frenchwoman who had a 18cm height advantage on the Australian.
But Gavrilova steadied and got the break back at 4-3 before comfortably serving out the opening set in 32 minutes.
She again got the early break in the second set before immediately giving it back which set the tone for proceedings.
The pair then traded breaks much to the frustration of the home crowd who were hanging off every ball which came off Gavrilova's racquet.
Eventually it was Mladenovic, herself one of the rising stars of the tour, who got the edge, breaking the Australian to take the set 6-4.
The 22-year-old was a quarter-finalist at last year's US Open and has reached the third round or better at all four majors.
Things continued in the same vein in the third set with breaks being traded before the pair settled in for one of the battles of the tournament.
Gavrilova's links to Australia started five years ago when she met South Australian Luke Saville at the junior Davis Cup in Mexico.
The pair started dating and the Russian moved to Melbourne permanently three years ago and she qualified for the main draw of a grand slam for the first time at the 2013 Australian Open, losing in the second round.
A knee reconstruction took out most of 2014 and she bombed out in the first round at last year's Open with her best Grand Slam result making the second round of the French Open.
But she had a bullet next to her name - Gavrilova was a finalist for the WTA Rising Star Award - with her giant-killing ways not a surprise to Tennis Australia who have known for quite a while they'd adopted a very special talent.
Gavrilova's describes Hewitt as one of her idols and she tweeted a message to the Australian great after his second-round loss: "It is sad to see you go @lleytonhewitt #Legend".
