In missing weight for UFC 225, Romero became the first fighter in the organisation's history to come in heavy for more than one title fight as the Cuban wrestler weighed in at 187.7lbs for his interim title fight with Luke Rockhold in February.
The rest of the fighters on the card also made weight for the Saturday, June 9, event.
Instead, Whittaker has the win and the belt, though neither seemed a sure thing as the fight came to its conclusion.
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UFC middleweight champion Robert Whittaker easily hit the mark, but after a hard cut, his opponent, Yoel Romero did not.
This marks the second time in a row that Romero has missed weight for a fight. In the fifth, Romero controlled the round, bring down Whittaker with a series of left hands, then pounded with a right hand.
Whittaker opened strong, immediately attacking Romero's lead leg with side kicks before establishing a solid jab.
There is no word from the UFC if the fight will proceed and how the purses will be divided.
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Renowned for producing explosive moments after being seemingly passive, Romero did just that in the third to change the momentum of the fight.
Whittaker, who claimed a unanimous decision over Romero last July, was asked earlier this week about the recent spate of missed weights among his UFC peers.
Romero landed several other strikes, but Whittaker survived and landed a few of his own in an epic round. Late in the round, Whittaker landed a jab that nearly instantly closed Romero's right eye, but he fought on to the horn.
This card featured five current and former UFC champions in front of a sold-out United Center crowd.
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Last year, though, having reached top spot in the world rankings she duly missed four successive major cuts. World number 97 Smith, winless in 222 starts on the LPGA Tour, failed to notch a single birdie.
In the co-main event, Colby Covington (-130) is a narrow, nominal favorite against Rafael dos Anjos (+110) as they vie for the interim welterweight title. All three judges scored the fight for Tuivasa via identical 29-28 scores. Anderson, who exited Invicta Fighting Championships on four straight wins, drops her Octagon debut, while Holm rebounded from her last bout, a decision loss to UFC featherweight champion Cristiane Justino.