One of the few men to defeat Manny Pacquiao says Jeff Horn would be "scared out of his mind" as the biggest fight of the Australian's life draws ever closer.

But Timothy Bradley Jr, who ended Pacquiao's run of 15 straight wins with a controversial split decision victory in 2012, believes Horn has a genuine chance in the 'Battle of Brisbane' -- provided he can overcome his nerves and put his powerful right hand to work.

A hometown crowd of 50,000 awaits on Sunday at Suncorp Stadium for Horn, who Bradley said had been handed a "lottery ticket" with his shot against Pacquiao, an 11-time world champion who has won belts in eight different divisions.

Horn is just 17 fights into his professional career and has never faced a top-level opponent like Pacquiao.

"I know how he feels right now, he's scared out of his mind. He is out of his element," Bradley, a four-time world champion from California, told reporters on Friday at a VIP luncheon event in Brisbane.

"Jeff (is) young, not a whole lot of experience, in the fight of his life, 50,000 people, there's a lot of pressure riding on him.

"But if he can put all of that aside, go in there and fight his gameplan... don't be reckless, and don't be dumb. Be smart.

"(If he can do that) he has a good chance of winning the fight. He has to hurt Manny Pacquiao with that big right hand he has."

Bradley, 33, has faced Pacquiao three times and lost twice -- most recently in April last year when he was defeated in their bout for the vacant WBO welterweight title, which the Filipino great will put on the line on Sunday against Horn.

Bradley said reviewing footage of Pacquiao was one thing but sharing a ring with him was quite another.

"It's kind of hard to explain, being in the ring with Pacquiao," Bradley said.

"At first you watch him on film but when you actually get in there and fight him, he's actually faster than you saw on film, his footwork is a lot quicker, he's a lot smarter.

"He's very deceiving. He's a legend for a reason and even at the age of 38 he still has all of his wits.

"But good luck to Jeff Horn and his camp.

"I think Jeff's going to do something special on Sunday with all the hard work he's done."