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The Mexican's challenge for the South African's IBO super middleweight title was based entirely on the forlorn supposition that he could gain a victory via a single lethal knock-out blow.
He made little or no effort to conceal this with a minimal work-rate of punches.
But the Sierra's record of securing 23 knock-outs from 25 victories in his 30 previous fights was accurate enough testimony that Oosthuizen could not take him lightly in spite of winning almost every round.
It, however, came as a complete surprise to everyone present when the referee stopped the fight in the 11th round.
The IBO's 24th-ranked Sierra might never have come close enough to decking Oosthuizen with his stinging, but sporadic punches.
The Mexican never looked like being knocked out himself despite the beating he suffered and the referee's decision to halt the contest was described by one official as "a brain storm."
It marred what was an engrossing, if unspectacular contest and the battle of attrition should have been allowed to continue towards the inevitable conclusion of an Oosthuizen points decision after 12 rounds.
As for Sierra, he was almost in tears over the referee's arbitrary intervention and remonstrated bitterly over the TKO decision.
But Oosthuizen demonstrated his superiority and growing maturity under wily trainer Harold Volbrecht.
He also showed why he had gained the deserving victory over the IBO's higher-ranked number nine contender, Aaron Pryor jnr, in his previous contest in the United States.
The five-bout programme billed as "The Grand Slam" was nothing of the sort, with Zack Mwerkassa and Joey Stiglingh gaining first and second round knockout wins respectively against under-strength opponents at the start of the programme.
Sipho Taliwe's WBC international junior lightweight win over Jasper Seroke was more of a contest and there was some belated action in the drawn fight between Samuel Malinga and Jason Bederman.
Results of the "Grand Slam" tournament at Emperors Palace:
Catchweight: Zack Mwekassa beat Kizito Ruhamya on first round KO.
Catchweight: Joey Stiglingh beat Voyolwethu Sirhayi on second round KO.
WBC International junior lightweight title: Sipho Taliwe beat Jasper Seroke on points.
Junior Welterweight: Samuel Malinga and Jason Bedeman in a draw
