Manny Pacquiao has warned Floyd Mayweather that he has rediscovered his killer instinct in time for their Las Vegas superfight.

Pacquiao was at the peak of his powers in 2008-2009 when he amassed four successive knockouts, including the notable scalps of Oscar De La Hoya, Ricky Hatton and Miguel Cotto.

Of his nine fights since crushing Cotto, the only one that failed to last 12 rounds was when he was flattened by Juan Manuel Marquez in 2012.

The Filipino southpaw enters his 65th fight as underdog for the first time since dismantling De La Hoya and he is revelling amid the widespread expectation that Mayweather will prevail at the MGM Grand.

"I'm so happy because the feeling of the killer instinct and the focus that I had years ago is back," Pacquiao said.

"I haven't felt like this in my recent few fights, but now I feel different. I'm eager to show something, especially because I'm the underdog.

"Every time I've been the underdog, like against Oscar De La Hoya, it's a different feeling and I'm so glad that it's back.

"This is one of the most important fights for my boxing legacy. I want to make this fight for my boxing legacy. I want to win, that's my goal."

The key area of debate in fight week has been what gameplan Pacquiao will use to inflict a first defeat on Mayweather, who has pointed out that every opponent he has faced has tried and failed to place him under sustained pressure.

The 38-year-old Las Vegas resident has yet to meet a boxer of Pacquiao's speed and relentless energy, however, and expectation is that he will be subjected to a sustained 12-round fuselage of rapid combinations.

"We've not been working on one strategy, we have two or three. If he wants to fight me, then that's good for me," Pacquiao said.

"But if he's running and running and moving around the ring, then we've prepared for that as well. I'm different to the 47 people he has fought before. I'm faster than them."

Pacquiao's coaching mastermind Freddie Roach outlined the plan of attack that he believes even Mayweather, with all his ring intelligence, will be unable to counter.

"We feel we have better speed and power and will be using that. We'll use lots of quick movements and I don't think Floyd be able to handle that," Roach said.

"I don't think his legs are there any more. He can't move for 12 rounds like he used to. He'll be running for 12 rounds but the thing is that we'll catch him at some point along the way.

"Floyd has put muscle on and people don't put muscle on unless they want to fight.

"He'll come out early and I hope that happens because we'll be well prepared for it. We'll put pressure on him.

"Manny is reckless, that's his style. He tries to win fights and wants people to enjoy them. He gives his all. Sometimes he gets knocked out and that's part of the sport.

"I'm not really impressed by undefeated fighters because losses make you a better person and fighter."