Boxing great Manny Pacquiao believes IBF welterweight champion Kell Brook has it all to do when he steps up two divisions to face fearsome middleweight king Gennady Golovkin at the O2 Arena on Saturday.

Both men will put their unbeaten records on the line in London after weighing in inside the 160lbs limit on Friday.

Pacquiao is preparing to fight WBO welterweight title holder Jessie Vargas on November 5 in Las Vegas after reneging on a short-lived retirement.

The 37-year-old Filipino icon made a habit of stepping up through the weight classes to dethrone bigger men during his heyday around the turn of the decade and fully appreciates the scale of the task facing Brook, with an astonishing 32 of Golovkin's 35 professional victories coming by way of knockout.

"Yes, I'm going to watch that fight. I'm excited about that fight," Pacquiao, an eight-division champion, told Omnisport.

"It's hard and tough for Brook. Triple G [Golovkin] is bigger and Triple G is stronger, so it's going to be hard for him.

"It's a good fight but it is so hard for Brook to fight Triple G."

Brook was expected to take on Vargas in a welterweight unification clash before deciding upon his audacious bid for greatness.

The 27-year-old American wished his would-be rival well and believes a bout between himself and Brook could still happen once their respective superfights are out of the way.

Vargas told Omnisport: "I signed my end of the deal. Whatever the reason was that he chose to fight Triple G instead of myself, only he could tell you that.

"But I wish him the best of luck. Everything happens for a reason and, whether he wins or loses this fight, it's still a fight that can be made in the future.

"First he has a tough task in front of him, a very tough task, and I have a fight ahead of me on November 5. It will be two good nights of boxing."

Nevertheless, Vargas took little time to ponder a prediction that does not bode well for the Sheffield fighter.

"Tripe G by seventh-round knockout," he ventured.