Whittaker (20-4) recovered from a hyperextended knee in the first round with a strong striking performance, peppering Romero (12-2) with punches and kicks over the final three rounds. The Australian won his eighth consecutive fight, earning the victory 48-47 on all three judges' scorecards at T-Mobile Arena.

"It's a moment I've always dreamed of," Whittaker said. "My knee was definitely hurt. I injured it in camp, and Romero's kick set it back weeks. I know that Romero will capitalize on any weakness he sees, so I had to play it off. That's just what champions are made of."

The main event of the UFC's traditional July showcase in its hometown was canceled earlier in the day when bantamweight champion Amanda Nunes was hospitalized. She was scratched from her second title defense against Valentina Shevchenko, who criticized the champion's preparation for the bout.

Alistair Overeem also beat Fabricio Werdum by narrow majority decision in the third career meeting of two veteran heavyweights, and former lightweight champion Anthony Pettis also returned to the division with a unanimous decision victory over Jim Miller.

Whittaker took advantage of his bout's promotion to the main event by claiming the belt in front of a pay-per-view audience. He earned the chance to fight for the full 185-pound title held for the past year by Michael Bisping, who is injured.

Bisping stepped into the cage afterward and congratulated Whittaker in his own irascible style.

"The fact that you're standing there with a belt on makes me sick," the Englishman said, throwing his own belt on the canvas. "Take that belt. Fight for it."

Romero's Olympic-level wrestling skills kept Whittaker on the defensive for much of the first two rounds. Whittaker increased his striking output while Romero sat back in the third and fourth, with Whittaker also defending Romero's takedowns more adroitly.

Romero was cut on his left eyebrow while Whittaker had a bloody nose and mouth in the final round, which looked even until Romero slipped onto his back with 90 seconds left after failing to land a strike. Whittaker jumped on top to rain down blows to the final horn.

All three judges saw the fight identically, scoring the first two rounds for Romero and the final three for Whittaker.

The 40-year-old Romero was gracious after his eight-fight winning streak ended, praising his opponent.

Whittaker will get the chance to remove the interim designation from his belt when Bisping returns from injury, although it might not happen this year. The middleweight division has been in limbo ever since Bisping shockingly beat champion Luke Rockhold last summer and then fought old rival Dan Henderson instead of Romero, Whittaker or another top contender.

For the third straight year, the UFC had to change the main event of its July show in Vegas at short notice. Last year, Jon Jones was sidelined by a violation of the promotion's anti-doping policy three days before UFC 200.

Shevchenko still showed up at T-Mobile Arena after being denied her long-awaited title shot when Nunes fell ill in the hours before their rematch.

Shevchenko was furious with Nunes, claiming the champion's drastic weight cut was responsible for her illness. She also echoed UFC President Dana White's statement that Nunes was medically cleared to fight, but declined to go from the hospital to the cage.

"You can't just say, 'I don't want to fight,'" Shevchenko said. "I couldn't believe at the very last moment that this would happen to me. Of course I am upset. ... I understand what she wanted. She wanted to cut (weight) in a very short time. You cannot do this. Our bodies need more time to recover."

Shevchenko was greeted with loud cheers when she was shown on the arena's video boards before the main event.

Overeem (43-15) and Werdum (21-7-1) finished a trilogy of fights between two of the most accomplished mixed martial artists of their generation. Werdum won their first meeting in the Pride promotion in 2006, and Overeem won their Strikeforce heavyweight rematch in 2011.

Overeem was slightly better during the first two rounds, and he survived the third after Werdum floored him with a knee to the head.

Pettis (20-6) returned to form in his return to 155 pounds, dominating the veteran Miller with flair and force. Pettis lost his title belt in March 2015 to start a 1-4 skid that included a two-fight stint at featherweight, but he looked sharp while carving up Miller (28-10), who bled copiously.

Before the pay-per-view portion of the card, heavyweight Travis Browne took his fourth consecutive loss in a second-round submission to 40-year-old Russian grappler Aleksei Oleinik.

Browne (18-7-1), Ronda Rousey's fiancé, lost for the sixth time in his last eight fights despite dropping Oleinik with punches in the first round.