Rune knocked out of Australian Open by wildcard Cazaux
Cazaux was not ranked high enough to even get into qualifying at Melbourne Park last year but outplayed the Danish eighth seed to record his second win on his Australian Open debut and set up a clash with Dutchman Tallon Griekspoor.
"It was a crazy match," said the French player. "Holger is a wonderful player, I knew it would be a big fight and I was ready. I'm so happy.
"Tennis is a show and I gave all I had in me."
Rune is regarded as one of the most promising talents in the men's game but for the first two sets at least it was Cazaux who looked the pick of the two youngsters wearing matching backwards white baseball caps.
The world number 122, riding a big serve that earned him 18 aces, clinched the first set on a tiebreak before a single break put him two sets up to the delight of a noisy band of his compatriots on Margaret Court Arena.
Rune, who took a medical timeout and received regular attention to his left thigh during changeovers, finally got a look at Cazaux's serve in the 10th game of the third set and broke to cut the deficit.
The aggressive Frenchman, who beat Laslo Djere over five sets for his first Grand Slam win in the first round, came straight back at the Dane with a break to open the fourth and Rune needed to fight hard to prevent Cazaux going 3-0 up.
Facing defeat, Rune threw everything he had at his opponent but Cazaux stood firm in the face of some ferocious shots and summoned up a brilliant backhand cross-court winner to break the Dane one more time and secure a famous victory.