Terence Crawford has become the undisputed welterweight champion after a dominant performance against Errol Spence Jr in Las Vegas.

Crawford unleashed a flurry of punches in round nine to a wobbly Spence who could not defend himself, forcing the referee to step in and stop the fight.

It took just two rounds for Crawford to land his first of three knockdowns, with the next two coming in round seven.

Spence was brave throughout the bout, hanging on in a fight he was significantly behind in, but eventually Crawford took it out of his hands.

Crawford showed his respect to his downed opponent after the fight and said he would be up for a rematch.

“It was a good stoppage. I was on the verge. The referee did what he was supposed to do to protect the fighter,” Crawford said.

“If the fight happens again I am pretty sure the support will come out again for both of us.”

Terence Crawford declined to commit to fighting Errol Spence Jr but is open to it after beating David Avanesyan with a sixth-round knockout shot to defend his WBO welterweight world title on Saturday.

Crawford was far too good for Avanesyan in Omaha, further enhancing the appetite for a unification showdown with IBF, WBA and WBC claimant Spence.

The 35-year-old southpaw reportedly agreed terms with Spence in September for a November fight which never materialised, while many expected Saturday's bout with WBO number six contender Avanesyan to be lopsided.

"Listen, I'm a free agent, this was a one-fight deal," Crawford said after Saturday's win when asked about the prospect of fighting Spence.

"Hopefully we can go [back] to the drawing board and these big fights come about in the near future."

Spence was involved in a two-vehicle accident on Saturday where he was not at fault but thankfully escaped relatively unscathed.

Crawford added about his rival: "I wish him well. I hope he’s alright - mentally, physically and emotionally. This is just a sport. We're all fighting for that number one spot, so it ain’t personal."

The American knocked Avanesyan out after a strong burst in the sixth round, landing a left-handed uppercut followed by a short left uppercut-right hook combination to seal victory.

The win meant the WBO world welterweight champion made his sixth straight defence and recorded his 10th straight knockout. It also improved his overall record to 39-0.