MACAU — Boxing is a hurt business, as Manny Pacquiao found out when he ended up face down on the canvas after being knocked out just three fights ago by Juan Manuel Marquez.

It’s also a risky business, and there are some in the sport who believe Pacquiao is taking a big one when he gets up Sunday morning in this gambling enclave for his welterweight title fight with Chris Algieri.

The stakes are always high anytime Pacquiao fights. This, though, is a fight he must not only win but look good while doing so if there is to be any hope of a megafight with Floyd Mayweather Jr.

“A fifth- or sixth-round knockout would be perfect,” promoter Bob Arum said. “But who really knows?”

Pacquiao is a heavy favorite against Algieri, a New Yorker who seemingly came out of nowhere to get the fight. But Algieri poses problems for Pacquiao with both his height — he is a half head taller — and a left jab that he used to come back from two first-round knockdowns to beat hard-hitting Ruslan Provodnikov in his last bout.

Algieri is also unbeaten in 20 fights and extremely confident his boxing skills will allow him to prevail in a bout being fought at a catch weight of 144 pounds.

“When I look into the mirror, I know I’ve done all the work necessary to win this fight,” said Algieri, who holds a master’s degree in clinical nutrition. “I’m sure that Manny realizes this will be a real fight.”

Pacquiao will make more than $20 million for the fight regardless of what happens, but an even bigger payday with Mayweather, a bout boxing fans have been clamoring for years. That would likely be out the window for good if Algieri finds a way to win.

“I’m not predicting a knockout, but I’m looking for a good fight and looking to prove I can still fight,” Pacquiao said. “I’m looking to maintain my speed and my power in this fight.”

Talk about a possible Mayweather fight heated up this week. Though Algieri is an attractive and articulate opponent who has done more than his share to sell the bout, the tantalizing possibility of a Pacquiao-Mayweather fight suddenly seems real again.

Arum said he has had talks with Les Moonves, the head of CBS Corp., to bring a previously reluctant Mayweather into the ring with Pacquiao next spring, and believes they could have a deal in place by the end of the year. Mayweather is under contract to the Showtime network, which CBS owns, for two more fights.

Algieri could ruin it all, though Pacquiao trainer Freddie Roach believes his Cinderella story will end quickly when the two finally get into the ring about noon Sunday for a bout that will be on pay-per-view Saturday night in the U.S.

“He (Algieri) doesn’t have the greatest chin in the world and we’re going to show that,” Roach said. “Manny has told me he will knock him out in one round. He won’t repeat that, but he has that kind of confidence in him. I feel great about this fight.”

Pacquiao’s knockout punch has been missing in action since he dispatched Miguel Cotto in the 12th round of their November 2009 fight. He’s lost two fights since — one a decision by Timothy Bradley that almost everyone ringside thought Pacquiao easily won — and may not be the same fighter who thrilled fans with big wins over the likes of Cotto, Oscar De La Hoya and Ricky Hatton.

But Pacquiao believes he has plenty left as he approaches his 36th birthday, and is eager to put on a show in a 13,000-seat arena that will include plenty of Filipino fans eager to see him show the punching power that helped turn him into a pay-per-view star.

“Not only the Filipino people, but all the fans around the world,” Pacquiao said about regaining his knockout ways. “Even me.”

Algieri, a former kickboxing champion who still lives in the basement of his parents’ home on Long Island, has become somewhat of a sensation in the New York area after his win over Provodnikov in June. He would become much bigger should he overcome the 7-1 odds and beat a fighter who is a national hero in his own country.

“I guess we’ll find out when I win on Sunday,” Algieri said. “That’s a cross I’m willing to bear.”