The 36-year-old Brazilian spent the first few minutes feeling his opponent out with his jab, then used his heavy right leg to batter Shamrock's thigh in the opening round of their main event fight in Sydney, Australia.
While it usually takes time for the damage from leg kicks to accumulate, it only took a few of Rizzo's slicing shots before Shamrock was left hobbled and helpless, forcing referee "Big" John McCarthy to stop the bout at 3:33 of the first round after the wounded Shamrock offered no response to a few perfunctory punches from a reluctant Rizzo.
The loss was Shamrock's sixth in his last seven fights, begging the question: how much longer will he insist on fighting? Judging by his post-fight remarks to Impact FC commentator and former UFC fighter Elvis Sinosic, as long as promoters are still willing to pay him and fans are still willing to watch him.
The rousing response that sentiment elicited from the Australian crowd probably didn't do much to convince the UFC Hall of Famer that it's time to hang up the gloves, but maybe Rizzo's leg kicks did.
Shamrock started off well enough in the opening moments, tossing credible punch combos and answering Rizzo's aggression with some of his own. But as soon as "The Rock" started to chip away at his thighs, the look on Shamrock's already bloodied face suggested he might rather be somewhere else.
Whether the beating he took from Rizzo – a fighter ten years his junior, and yet a fighter whose best days are also behind him – will finally convince him to find another way to occupy his time that doesn't involve so much pain and suffering, we'll have to wait and see.
If there's one thing Shamrock has shown that he's not afraid of over the course of his storied career, it's taking a beating in exchange for a paycheck.
Elsewhere on the card, UFC cast-off Paul Daley fought a back-and-forth battle with Daniel Acacio in the first two rounds, displaying superiority on the feet but doing little more than surviving when the fight hit the mat. In the third frame, however, a salvo from Daley stunned the game Acacio, who tried for a takedown and then settled for pulling guard.
Sensing the end was near, Daley launched a elbow that opened a considerable cut on Acacio's scalp. Acacio put a hand to his head to check for blood and, when he found there was plenty, verbally submitted a few seconds later.
It was one of several somewhat anti-climactic finishes, as middleweights Paulo Filho and Denis Kang fought to a split draw, and Murilo Bustamante quit under bizarre circumstances midway through the second round against Jesse Taylor, apparently suffering from a sudden loss of equilibrium.