Cena rapidly climbed the ranks, and he began to develop an edgy mean streak as well.
By In April 2003, Cena defeated the likes of Eddie Guerrero, Undertaker and
Chris Benoit to become the No. 1 Contender for Brock Lesnar’s WWE Championship.
Cena found himself in his first main event, facing Lesnar at Backlash. Cena came up
short, but it was clear that he was starting to make a name for himself.

During this time, Cena, who also became known as the Dr. of Thuganomics, started to
show the world his unique ability to be able to rattle off impromptu freestyle raps,
running down his opponents week after week. Cena’s masterful rhymes would catch the
   ire of his opponents, taking them off their game and giving himself a mental
advantage before the match even started. 

By the time WrestleMania XX rolled around, Cena was reaching the top of his game.
He had the daunting task of being in the opening match of the biggest show of the
Year and to make the situation even tougher, he was going head to head against the
   largest athlete in the world – Big Show. It was Cena’s biggest (literally and figuratively)
challenge to date, and Big Show’s United States Championship was also on the line.
Cena was certainly the underdog, but with the capacity Madison Square Garden crowd
behind him, he was able to hoist Big Show up for an improbably FU and his first taste
of gold in WWE. Cena became the United States Champion.

Cena took on all challengers as the U.S. Champion, and one of his toughest challengers
was the five-time WCW Champion, Booker T. Cena and Booker T’s rivalry became so
heated, it could not be decided in just one match. Instead, General Manager
Theodore Long ordered they have a Best of Five Series to settle things once and for all.
The series went to a deciding fifth match, where Cena was able to overcome Booker at
No Mercy 2004 to retain the United States Championship.

But just a few days later on SmackDown, Cena ran into newcomer Carlito. It was
Carlito’s debut match, but the newcomer somehow managed to pull off the huge
upset win, becoming U.S. Champion in his first ever match. Cena would have to wait
for his rematch, though, as he suffered a serious kidney injury in a bar fight
later that same night. It was later learned that Carlito’s associate, Jesus, was the
assailant. But nonetheless, Cena was out of action for more than a month.