David Price missed out on a major upset as he was knocked out by former world WBA heavyweight champion Alexander Povetkin at the Principality Stadium.
The Liverpool fighter, 34, was knocked out in the fifth round by the Russian - who has twice failed drug tests.
Povetkin, 38, floored him with a brutal left hook on the undercard of Anthony Joshua's win over Joseph Parker.
Earlier, there were wins for WBA bantamweight champion Ryan Burnett and lightweight Anthony Crolla in Cardiff.
Price, who enjoyed a height and reach advantage, landed two shots at the end of the first round to give himself hope, having come into this contest as a 20-1 outsider.
But Povetkin's power told in the third as he chopped down Price with a booming left hook that cut him just above his left eye.
The Olympic and Commonwealth Games medallist answered the count and gave Povetkin plenty to think about at the end of the round with a huge right hand of his own, but he could not sustain that scare.
This fight was never going the distance and Povetkin's pedigree told in the fifth round when he floored Price with a devastating left under the chin after a combination opened Price's defences.
The ringside medics attended to the Olympic medallist, but thankfully Price was able to leave the ring under his own steam.
Povetkin, the mandatory challenger to face the winner of Anthony Joshua v Joseph Parker, could now face another Matchroom fighter in Dillian Whyte.
"Povetkin is mandatory for Anthony Joshua, but Dillian wants to fight him in June," Eddie Hearn said after the fight.
Burnett wins despite broken hand
Northern Ireland's unbeaten WBA bantamweight champion Ryan Burnett advanced his record to 19-0 by beating Venezuelan Yonfrez Parejo but suffered a suspected broken hand in doing so.
Mandatory challenger Parejo had only been defeated twice in 24 contests, but Burnett, who has relinquished the IBF title he won by beating Lee Haskins last year, was far too busy and dominant for a boxer fighting in Europe for the first time.
Parejo is a class operator and gave Burnett plenty to think about in the opening rounds with the Belfast boxer switching to southpaw in the third session as he looked to control the centre of the ring.
However, Parejo has only failed to go the distance once in his career and proved a tough nut to crack, with Burnett doing all he could to entertain the crowd by constantly dropping his hands and looking to force Parejo to throw and miss.
The Venezuelan never let up and did all he could, but Burnett's superior speed and footwork was always in evidence as he successfully defended the title he won against Zhanat Zhakiyanov.
Burnett secured a unanimous decision, with the judges scoring the contest 120-108, 120-108 and 116-112.
Burnett is now likely to face the winner of Jamie McDonnell's fight with Naoya Inoue in Japan in May with the WBA keen to ensure that is Burnett's next defence. However, the Belfast boxer might need to heal first after damaging his right hand in the third round.
"I think I broke my hand so I had to rely on my boxing skills," Burnett told Sky Box Office.
Crolla wins to reignite world title hopes
Anthony Crolla's up-and-down 2017 carried on into 2018 as he won against a game Edson Ramirez in a fight that was never as straightforward as it might have been.
Following a year in which Crolla lost to Jorge Linares to give up his WBA world lightweight title before beating Ricky Burns, he recovered from a cut suffered in a clash of heads in round four to comfortably win a unanimous points decision over 10 rounds.
However, this was a scrappy contest with a crowd used to watching Crolla fight at close quarters instead forced to watch him box off the back foot as he controlled the contest with his jab and listened to his trainer Joe Gallagher who told him to box clever.
Crolla took the decision 100-91, 100-90 and 98-92 and will now look to get back into the world title picture.
Joshua v Parker undercard: Round-up
Sunderland's Josh Kelly continues to race through the paid ranks after claiming his first title in his sixth fight as he impressed against former world champion Carlos Molina, beating the renowned Mexican on points over 10 one-sided rounds.
The 24-year-old had never fought beyond six rounds before but the durable Molina gave him a decent examination with Kelly always the busier and stronger boxer who produced some eye-catching combinations, taking a unanimous decision.
Another debutant title winner was Joe Cordina who ensured the Welsh fans will have left happy by claiming his first professional title by winning the WBA international lightweight championship.
The 'Welsh Wizard' was always in control against stand-in opponent Hakim Ben Ali in his first scheduled 10-rounder, only needing three rounds as he stopped his Belgium opponent after three successive knock-downs, which was enough action for the referee. Cordina looked especially vicious focusing on big body shots and will now look for a step up in class with his opponents.
"I would have liked more rounds, but I was happy with my performance, I wasn't trying to force the stoppage," Cordina told BBC Sport Wales after his fight.
Another Welshman, Morgan Jones, boxing out of Aberdare via Cape Town, lost the opening bout of the night and his undefeated record in a fight that saved a flurry of action of for the final session. Jones floored Mose Auimatagi Jr and looked set for a sixth stoppage victory, but instead got caught with a solid right hand and was stopped after an onslaught of shots with just eight seconds remaining.
Light-heavyweight prospect Josh Buatsi earned a comfortable points' win over Bartolmiej Grafka to extend his unbeaten record to five.