Floyd Mayweather Jr. has made his New Year’s Eve plans official.

Mayweather will exit retirement once again and fight 20-year-old Japanese kickboxing phenom Tenshin Nasukawa on Dec. 31 at the 37,000-seat Saitama Super Arena in Saitama, Japan, about a half-hour drive outside of Tokyo.

Mayweather and Nasukawa appeared at a news conference in Tokyo on Monday (Sunday night United States time) that was streamed worldwide on YouTube to announce the fight with officials from the Rizin Fighting Federation, a Japanese promotional company founded in 2015 that puts on MMA and kickboxing events.

Although the fight was announced, several key details were not, including the specific rules of the bout, the number of rounds, the weight limit, how the fight broadcast would be distributed and what type of gloves they would wear. Earlier in the day, Mayweather posted a photo of himself on social media wearing Rizin MMA gloves.

Before the announcement, Mayweather posted a photo on Instagram of himself wearing Rizin gloves with the caption “December 31st 2018 #Tokyo.”

Mayweather, who makes it a point to regularly bring up how much money he will make for his fights, did not discuss his pay for the event. In fact, he downplayed the financial aspect of the deal.

“As far as the weight class and the rules, we’ll talk about that,” said Mayweather, who was dressed in a red, white and blue sweat suit and flanked by One Entertainment’s Brent Johnson, who represented him in making the deal with Rizin. “We’ll get that situated within the next couple of weeks. This particular bout is a special bout as far as we’re giving the people something they’ve never seen before. The world has never seen Mayweather compete live in Tokyo. Throughout the years, I’ve seen a lot of fans from Japan come to Las Vegas and come to the U.S., but me and my team said let’s do something different. Let’s go over to Japan.

“As far as the weight class, we’re not really worried about that. It’s all about me going out there and displaying my skills against another skillful fighter. So we both want to go out there and display our skills and give people excitement. I just want to entertain. I will speak with my team and I will let my team get with Rizin and I’m pretty sure they’ll come up with a great idea, but it’s all about entertainment and that’s what we want to do for the people.”

Rizin chairman Nobuyuki Sakakibara, who used to run the now-defunct Pride Fighting Championships promotional company, said they were still working out some of the details.

“We still have more work cut out for us, but we think it will be a great fight everyone will be excited to see,” Sakakibara said through a translator.

Mayweather (50-0, 27 KOs), 41, of Las Vegas, came out of a two-year retirement in August 2017 and knocked out UFC superstar Conor McGregor in the 10th round of a massively hyped boxing match in what became the second-highest grossing event in combat sports history behind only Mayweather’s 2015 victory over Manny Pacquiao.

In September, Mayweather said he planned to come out of retirement again and announced he would face Pacquiao in a rematch in December. While Mayweather has said he still plans to fight Pacquiao again, that fight likely would not happen until at least the spring. Pacquiao plans to defend his secondary welterweight title against Adrien Broner in January.

In the meantime, Mayweather will take on Nasukawa, who is 27-0 with 21 knockouts in kickboxing and 4-0 with two knockouts in MMA. He recently defeated former UFC fighter Kyoji Horiguchi in a kickboxing fight.

In boxing, Mayweather, who has won world titles in five weight classes from 130 pounds to 154 pounds, has been primarily a 147-pound welterweight for many years, though his fight with McGregor was a 154-pound bout.

Nasukawa, a southpaw, is considerably smaller than Mayweather. He holds the Rizin featherweight title, which is at 126 pounds.

“When I was offered this, I accepted right away without any delay whatsoever,” he said through a translator. “I would probably say this is my biggest event in life so far, and I’m very happy this is going to materialize. I’m a little surprised myself. Nobody has defeated my opponent in the past, and I’d like to be the man who makes history. My punch can change history. I will show that to you, so please stay tuned.”

When asked about the sort of rules he preferred for the bout, Nasukawa said he really didn’t care.

“I’m not particular about the rules, with or without kicks,” he said. “Boxing rules I wouldn’t mind. My opponent looks taller than he actually is, but I think I could hit him.”

Mayweather was complimentary of Nasukawa.

“He’s undefeated, so it’s obvious he’s been doing something right to be where he’s at,” Mayweather said. “He’s young, fast, [has] youth, undefeated like myself, and what the people want is excitement, and that’s why I chose Japan. You guys know about boxing, you guys know about fighting. I’m not really worried about the rules. I’m pretty sure that will get sorted out in the next couple weeks. My thing is just coming over here to display my talent and put on a show for the people.

“It’s always been a goal of mine to go outside the U.S. and go into another arena and display talent as a professional because I did it as an amateur.”

The fight will be Mayweather’s first outside of Las Vegas since he knocked out Sharmba Mitchell in the sixth round in November 2005 at the Rose Garden in Portland, Oregon.