LOS ANGELES (AFP) — Errol Spence and Terence Crawford chase history on Saturday in Las Vegas, where one will emerge as the first man to claim the undisputed welterweight world title in boxing's four-belt era.

The American rivals are both undefeated. Spence, the World Boxing Council, World Boxing Association and International Boxing Federation world champion, brings a record of 28-0 with 22 knockouts to the highly anticipated bout.

Crawford, holder of the World Boxing Organization belt, is 39-0 with 30 knockouts. The former lightweight and undisputed light welterweight world champion is riding a wave of 10 straight knockouts, including seven straight in his first seven fights at welterweight.

"It's going to be even sweeter to win the undisputed championship for the second time," the 35-year-old Crawford said upon arriving in Las Vegas this week.

The Nebraska native launched his welterweight run with a stoppage of Manny Pacquiao conqueror Jeff Horn in 2018 for the WBO title.

In his latest bout, Crawford knocked out David Avanesyan in the sixth round in December. No opponent has taken Crawford the distance in more than six years.

That victory paved the way for the long awaited meeting with Spence, who returned from a torn retina that quashed a planned bout with Pacquiao to punish Yordenis Ugas on the way to a 10th-round technical knockout in April of last year.

That marked Spence's second comeback from injury, the first coming when he was sidelined for months after a 2019 car crash.

Among the many who have longed to see the two meet is legendary trainer Freddy Roach.

He gives the edge to the "more complete" Crawford, but noted Spence's perfect record against a stronger list of opponents.

Spence has pointed to the quality of his opponents in shrugging off pundits who make him the underdog.

He traveled across the Atlantic to stop Kell Brook in the 11th round to claim the IBF title in 2017. In 2019, he seized the WBC belt with a victory over Shawn Porter.

"Every belt I've gotten I've taken from somebody," Spence said.

"I beat champion after champion. The guys with the belts have a lot more to fight for."

Spence also insists he's simply better than Crawford, and he's aiming to prove that on Saturday.

"I'm the better fighter. I'm better physically, mentally. I'm more durable than him. My skills are superior."

Crawford, not surprisingly, begs to differ.

"Terence Crawford is the best fighter in the world today, and that's just what it is," Crawford said Thursday at the final pre-fight press conference.

"This is the Terence 'Bud' Crawford era," he added.

"When you look at the body of work that I've done and each weight class that I've been in you can't deny that this is my era."

Spence plans to make it his own on Saturday in a fight he believes fans will be talking about "30, 40 years from now."

"I guarantee you it's gonna be nothing but fireworks from start to finish," Spence said.