Though most combat sports fans aimed their attention on UFC 297 on Jan. 20, Natasha Jonas and Mikaela Mayer headlined the biggest boxing card of the evening.Jonas, 39, entered the fight as the IBF world welterweight champion and aimed for her first title defense. In an eventful main event, Jonas defended her belt with a split decision victory over Mayer. View this post on Instagram Instagram PostThe result was not without controversy, however, as many fans did not agree with the decision. Supporters of the fan-favorite Mayer expressed their reaction to the bout on social media, with many coining the fight as a "robbery."Some upset fans even claimed the bout was "rigged" in favor of the champion.Fan reaction to Natasha Jonas defeating Mikaela Mayer [via @skysportsboxing on Instagram]The decision somewhat overshadowed the action in the fight, which gave viewers nonstop volume from start to finish.Regardless, the consensus opinion on social media appeared to be in favor of Mayer as some fans chimed in as Jonas supporters still claimed a faulty decision.Other fans commented:"Undisputed robbery 😂""I'm English but Mayer won that comfortably""Robbed @mikaelamayer 100%""Hmm I don't know about this one, Mayer looked great""Mayer won by miles"View other fan reactions to the Natasha Jonas vs. Mikaela Mayer result below: Fans reacting to Natasha Jonas' win over Mikaela Mayer [via @skysportsboxing on Instagram]Natasha Jonas vs. Mikaela Mayer full card resultsWith Natasha Jonas retaining her IBF world title in the main event of a Jan. 20 fight card, seven other fighters also got their hands raised in victory.Jonas was also not the only champion on the card to leave the arena with the same belt, as British super middleweight Mark Jeffers also retained his English super middleweight championship. Jeffers claimed to have broken his hand in the second round after the fight on social media. View this post on Instagram Instagram PostNatasha Jonas vs. Mikaela Mayer results: Natasha Jonas def. Mikaela Mayer by split decisionZak Chelli def. Jack Cullen by unanimous decisionMark Jeffers def. Germaine Brown by unanimous decisionKarriss Artingstall def. Lila dos Santos Furtado by pointsAaron McKenna def. Mickey Ellison by sixth-round TKO (2:21)Jack Massey def. Steve Eloundou Ntere by first-round TKO (1:57)Mikie Tallon def. Adam Yahaya by pointsSte Clarke def. Vasif Mamedov by third-round TKO (1:53)