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Two of mixed martial arts' greatest-ever fighters return to the octagon at UFC 239 as Jon 'Bones' Jones and Amanda 'The Lioness' Nunes put their world titles on the line in the two headline bouts of the evening.

The event is the main attraction of the UFC's annual International Fight Week celebrations, and in Jones and Nunes they are showcasing arguably the greatest pound-for-pound male and female fighters in the sport.

Jones returns to action in the main event as he looks to defend his UFC light heavyweight title against power-punching Brazilian Thiago 'Marreta' Santos.

Jones has looked virtually untouchable at 205lbs, but against Santos he faces a new, aggressive challenger who threatens to derail his reign.

Santos, whose nickname translates as 'sledgehammer' moved up to the light heavyweight division last September and reeled off a hat-trick of stoppage wins against Eryk Anders, Jimi Manuwa and Jan Blachowicz to earn himself a shot at the gold.

Jones says he's ready for Santos' power

Defending champion Jones is well aware of the Brazilian's knockout power, but says he is confident of overcoming that threat on Saturday night.

"I'm not too concerned with his power," Jones told reporters and fans at the open workouts held earlier this week.

"I think every man and woman in this audience is strong enough to knock me out if I let them punch me squarely in the chin.

"Every UFC fighter I've ever fought has had the power to knock me out. It's about being skilled enough to land that punch. Thus far, no one's been skilled enough to land that punch and I don't think anything is going to change on Saturday night."

Santos believes he is the man to do just that, however, and after his workout he boldly declared that he would dethrone Jones on Saturday night.

"This is my moment, my time. I will shock the world," he said.

"I will use my footwork, my power, and I will put pressure on him. Next Saturday, we'll all see the new light heavyweight champion of the world."

Nunes aims for history, Holm aims to shock the doubters

Nunes made history by becoming the first female fighter to hold two world titles simultaneously when she stopped the previously dominant featherweight champion Cris 'Cyborg' Justino at UFC 232 last December.

And the Brazilian arrived here in Las Vegas full of confidence following that career-defining victory as she bids to complete a set of victories over every woman to ever hold UFC gold at 135lbs and above.

"After the best fight of my career, I feel like I'm ready for everything," she told fans and reporters at the open workouts in the build-up to Saturday's bout with Holly Holm.

"Holly's a tough fight, but I feel like I'm way in front of her in this game.

"I'm the best, and I will prove it on Saturday."

Former champion Holm, who spectacularly dethroned Ronda Rousey with a head-kick knockout at UFC 193 in 2015, is bidding to reclaim the title with a win over betting favourite Nunes. And, speaking at the open workouts, the former multi-time world boxing champion said victory would be no shock to her, even if it might be to others.

"I understand that other people might think this is a shock, but for me to come in here, we really believe we can do this," the American said.

"So to win this fight isn't going to be shocking to us. But I hope the people who believe I can't do it, I hope it shocks you."