LAS VEGAS -- The biggest fight in boxing is on, and fans do not have to wait five years for it, like they did for Floyd Mayweather and Manny Pacquiao to finally meet.

Sure, it took time, but unified middleweight world champion Gennady Golovkin and Canelo Alvarez -- perhaps boxing's biggest star -- will finally meet for Golovkin's unified titles in the showdown that has been in popular demand for more than year.

The surprise announcement of the bout being signed came moments after Alvarez routed Julio Cesar Chavez Jr. -- 120-108 on all three scorecards -- in their bitter Mexican rivalry fight on Saturday night before a sold-out crowd of 20,501 at T-Mobile Arena.

Alvarez-Golovkin will take place Sept. 16 -- on Mexican Independence Day weekend -- at a site to be determined.

"I've already had several calls from around the world wanting to stage this fight," Golden Boy Promotions CEO Oscar De La Hoya said at the postfight news conference. "I literally had a missed call from Dubai, a missed call from the U.K. There's interest all over the world. Those negotiations, those talks, will start in the weeks to come. This is the most anticipated fight, right next to Floyd Mayweather-Manny Pacquiao, but the difference is this fight will have a lot of action, nonstop action."

Last May, De La Hoya and Alvarez said they would attempt to make the fight but then ducked it; Alvarez even vacated his 160-pound belt to avoid a mandatory with Golovkin, who held an interim belt at the time to go with his full titles and inherited Alvarez's belt.

Now a year later, they are going to get it on. Alvarez, who faced Chavez at a contract weight of 164.5 pounds, will fight at 160 pounds against Golovkin after refusing to fight his previous middleweight title bouts heavier than 155 pounds.

"They stayed true to their word that we'd make this fight for September 2017, and here we are making the announcement," said K2 Promotions managing director Tom Loeffler, Golovkin's promoter.

Alvarez (49-1-1, 34 KOs), 26, who also is a former two-time junior middleweight world titleholder, said he did not have a hard time staying focused on Chavez, even though the bigger fight with Golovkin (37-0, 33 KOs), 35, was signed and a secret.

"First, I consider myself a person of strong mental strength, and I knew I had to focus. First things first: I knew I had to win this fight, and we showed it," Alvarez said.

Golovkin, a native of Kazakhstan living in Santa Monica, California, and Alvarez were thrilled the fight was finally made.

"I'm happy to give the fans another great fight, which is the only thing I want to do in my career," Alvarez said. "The styles are there, they should mesh together -- and on my part, that's what you'll see."

Golovkin, who has made 18 title defenses, is coming off a tight decision win against Daniel Jacobs on March 18 and perhaps looked a little vulnerable.

"Everyone understands this is a tough fight for both of us," said Golovkin, appearing with Alvarez at the news conference after he had answered questions about the Chavez fight. "This is amazing fight for everyone. Of course, I respect his team. Everybody wanted this fight."

De La Hoya said that despite the starts and stops in negotiations, it was not ultimately that hard to get it across the finish line.

"The Triple G side is a pleasure to work with," De La Hoya said of Golovkin's camp. "We negotiated back and forth several times. No big deal. These big fights take time to make. It took a couple of weeks to make [once talks resumed]. We did start negotiations a while back and halted negotiations for about a month, and then we started negotiating two weeks ago, and it got done a few days ago.

"We are extremely proud we can have this fight with Triple G for you in September, which will be one of the biggest fights anticipated in recent years. When you have fighters who want to make the best fights, it makes the job much easier. Boxing is back, bigger and better than ever. You have your ups and downs. It's a roller coaster. Right now, we're living a great moment for boxing, and we will for a long time."