At UFC 121, Brock Lesnar lost his UFC Heavyweight title to Cain Velasquez in the first round of the fight. Brock Lesnar was upset after the bout and was quoted saying that he was ‘a bit delirious’. As he left the octagon and was on his way to his dressing room, Brock Lesnar was confronted by his WWE rival The Undertaker, who was at the Honda Center to watch the fight with his wife Michelle McCool.
The Undertaker was being interviewed by MMA expert/journalist Ariel Helwani and was recorded saying, "You wanna do it?" to Brock Lesnar as the current WWE Champion was walking past him with his MMA team. Brock Lesnar looked at The Undertaker and didn’t say a word at that moment. However, Ariel Helwani reached out to Brock Lesnar later where 'The Beast' talked about The Undertaker.
Brock Lesnar started the interview saying that after the match with Cain Velasquez, he was a bit delirious and was not able to hear what The Undertaker was saying. Brock Lesnar then said that he has always loved working with Mark Calaway (The Undertaker) in the WWE ring, but whatever he said after UFC 121, angered him. Brock Lesnar said that he didn’t know what The Undertaker meant when he said ‘You wanna do it?’.
"I don't know what he wanted to do. Maybe he was trying to fight. I don't know if he was there to fight or what or if he was there to get some attention or what, I don't know,” said Brock Lesnar.
Brock Lesnar said that The Undertaker holds a grudge against him because pro-wrestlers can’t do the things he did. He said that he can always go back to WWE, but Undertaker and the others can’t enter the UFC octagon. Brock Lesnar ended his answer saying that he is still confused about Undertaker’s question.
After returning to WWE in 2012, Brock Lesnar faced The Undertaker in three major PPVs - WrestleMania 30, Hell in a Cell and SummerSlam. At WrestleMania 30, Brock Lesnar became the first superstar to break The Undertaker's two-decade-long Wrestlemania winning streak. The Undertaker then took his revenge as he defeated The Beast at SummerSlam. But, Brock Lesnar picked up another win over The Undertaker at Hell in a Cell.