Horn believes their latest mind game - which forced the champion to weigh in three times - was a ploy to take his world title before he even got in the ring.

Horn (18-0-1) missed weight on his first attempt to leave his World Boxing Organisation welterweight title defence against Crawford (32-0) in jeopardy.

Shock reverberated around the MGM Grand Garden Arena when the champion came in at 148 pounds (67.13kg) when he first stepped on the scales. Horn believed he had made weight when he tested himself on Top Rank's official set before coming on stage.

The 30-year-old dropped his clothing and stood behind a sheet but still weighed 147.5 pounds (66.9kg) - just north of the division's 147-pound (66.67kg) limit.

The champion was given one hour to shed half a pound or forfeit his championship - so he jumped in a hot shower for a few minutes and returned to the scales.

He weighed 146.6 pounds (66.45kg) and wasn't flustered at all with the bout to go ahead on Sunday (AEST) with gold on the line.

"I wasn’t stressed because I know that’s exactly what their team wanted. I suppose someone has dodgied up the scales for us to be bang on or even underweight," Horn said.

"I think there was a bit of tricks up their sleeve with this one because he was just underweight and I was just over. I thought I was under actually.

"There was a bit of play with the scales, we thought we had calibrated our scales to the correct weight but they tricked us.

"I think they think I’m mentally weaker than I actually am but this stuff is all a part of it, I know it."

The weigh-in marked the final time Horn and Crawford will come face to face before the bell rings for the start of round one.

The challenger had adopted a brash approach in the lead-up to the bout having had plenty handed to him on a platter - he got the horsehair gloves he wanted, he got the original bout postponed for a hand injury Horn's camp labelled "fake", and he is staying in a lavish multi-level apartment at the MGM Grand.

He confidently put his hands on the belt during the face-off but Horn has noticed a chink in his armour.

"He was a bit shaky, yesterday he wasn’t. He was calm and collected, nice and still. Today he was very shaky and it seemed like something was off," Horn said.

"I knew this was coming, I knew they had plenty of tricks in their book. I can see face to face with Terence he was a bit rattled, a bit shaken."

Top Rank official Brad Jacobs admits the promotion is desperate for a Crawford victory but shut down claims the scales had been tampered with.

Jacobs says the "ridiculous" allegations can't hold up because "the scales are calibrated by the Nevada State Athletic Commission".

Horn's trainer Glenn Rushton maintains Horn has not been rattled by Top Rank's alleged foul play, with Duco Events promoter Dean Lonergan laughing "if Jeff was any more laid back he'd fall over".

Rushton believes the champion will overcome his latest hurdle and be ready for the biggest fight of his career come fight night.

"It doesn’t mean anything. He has made weight and he has got a fight on Saturday night here, Sunday in Australia," Rushton said.

"We’re ready, nothing will derail us. We’re ready for the fight of our life and Terence better be ready too because we mean business."

HORN v CRAWFORD

Sunday: Jeff Horn v Terence Crawford for the WBO welterweight championship at the MGM Grand Garden Arena, Las Vegas. TV: Live on Main Event from 10am.

*Caden Helmers is in Las Vegas as a guest of Duco Events.