LONDON- - Conor McGregor should rely on the skills that have made him a two-weight UFC champ and not try to out-box unbeaten American Floyd Mayweather when the two clash in Las Vegas on Saturday, former sparring partner Steve Collins Junior told Reuters.

The Irish mixed martial arts fighter turned up at the Dublin gym run by Collins Jnr's uncle Paschal about two years ago looking to hone his boxing skills, and the 27-year-old light heavyweight was happy to go toe to toe with him.

"I arrived early as my uncle told me to come in to do some pad work before the place opened and Conor came up. He must have cleared it with my uncle as he had the place to himself, he had the gym closed off," Collins Jnr said in a telephone interview.

"He started off sparring a guy for a round or two but the guy couldn't keep up with Conor's pace, so my uncle asked me if I'd spar him. I said fine, and got in there for five, six, seven rounds," he explained.

"Conor didn't care about the size difference between us, he just kept going. He barely took a break in between rounds, he'd just take a sip of water or whatever."

The Collins family are as close to boxing royalty as it gets in Ireland, with Steve Collins Senior a former WBO middleweight and super-middleweight world champ, and uncle Paschal a renowned boxing coach.

Collins Jnr, who has 10 wins, one loss and one draw in his 12 professional fights, was impressed by the offensive skills of the 29-year-old McGregor, who has never fought as a pro boxer.

"As an MMA fighter his boxing skills were brilliant - great stance, great balance, great shot selection and timing, he judged distance very well, superior to most MMA fighters."

However, the boxer did find chinks in the armour of the UFC lightweight champion.

"His only weakness really was his defence. I didn't find it to hard to land leather on him, and if you've got a guy like Floyd, he may find it even easier. If I had to pick one thing, it was that his defence wasn't brilliant," he said.

The boxing world has scoffed at the notion that McGregor can bring himself up to Floyd's level in the space of a few weeks, but Collins says he may not have to.

"I think he'll have to rely on what he already knows. There is so much to learn in boxing, even Mayweather is still learning, so Conor has to work with what he has.

"It takes years to be a complete fighter, and it will all be new to him on the day," he said.

Mayweather, who has not fought in nearly two years, needs a win to surpass Rocky Marciano's record and reach 50-0 for his career, while McGregor, who is making his professional boxing debut, is 21-3 in mixed martial arts.

Currently in training for a fight in Belfast in September, Collins Jnr says he will watch the fight but he won't travel to Las Vegas or stay up into the small hours to do so.

"If I was on Mayweather or McGregor money I'd go to Vegas, but I'm not!

"I'm in training so I can't afford to stay up all night, but we're going to record it and watch it together at the gym the next morning," he said, adding that he expected an exciting fight.

"Things Conor never expected are going to happen to him in the ring, and how he reacts to that will say a lot about him."