United States superstar Michael Phelps has refused an offer from men's 100m butterfly rival Milorad Cavic to buy him a new swim suit and said he'd give the Serbian one of his own instead. Cavic wears one of the new so-called supersuits made of polyurethane by manufacturers such as Arena and Jaked while Phelps has been sticking to a supposedly slower LZR suit made by Speedo, his sponsors. The two are the favourites for the 100m butterfly title, a race the produced so much drama at the Olympics in Beijing last year as Phelps won one of eight gold medals despite Cavic seemingly touching the wall ahead of him. Cavic has been fastest in qualifying for the final at the World Championships here and in the semi-finals he broke Phelps's world record of 50.22sec, set earlier this month, with a new marker of 50.01sec. Much of the talk surrounding this race has been about how much of an advantage Cavic's suit will be against Phelps and on Friday morning the Serbian, wary that if he wins such talk could tarnish his achievement, offered to buy the American a new suit so that they could both swim in polyurethane. 'If Mike wants an Arena, he just has to say it,' Cavic said. 'If he wants a Jaked and they don't want to give it to him free, I'll buy it for him. He has options. 'I think in the media it has been portrayed that he has no option, he has to swim for them (Speedo). It's a complete lie.' But Phelps replied that if anyone should change suits, it should be Cavic. 'I'm wearing this (a LZR), if he wants to wear a different suit he can wear this one,' said the American before looking ahead to Saturday's final. 'He (Cavic) was out quick, I know tomorrow if I want to be in that race my first 50m is going to have to be a lot closer than a second behind. 'He's got a lot of speed, it's going to be the same thing as it was at the Olympics. 'At the lowest I'm going to have to be at least at his hips to be able to run him down. 'He was at least 0.7sec faster (in the first 50m) than he was coming back. 'The big thing is getting some rest tonight and being able to step on the gas a little bit more really in that first 25m because that's when he really accelerates.' Cavic said he would like to ditch his polyurethane suit but only if the whole field does too. 'I think there's three options for Michael,' he said. 'The first option is to use the suit that he's wearing. 'The second option is to get one of these suits, which I guarantee Arena will provide him within the hour, as soon as he wants. 'The third option would actually be a dream of mine, to have the whole final with everybody swimming in briefs. 'I swear to God, this is it, this is what I want, but this is the most unrealistic of all scenarios.' Phelps, though, is supremely confident that he can still win in a LZR. However, he's had mixed evidence of his prowess in the slower suit at these championships. He was beaten into second place by German Paul Biedermann in the 200m freestyle, losing his world record in the process, but he won in a new world record time in the 200m butterfly. 'I was talking today about how only two (world) records have been broken in LZRs. Me and (Ryan) Lochte (in the 200m medley) so I think that's kind of cool,' said Phelps. 'It's going to be cool come January 1, being able to have all of us wearing pretty much the same suit and being able to talk about swimming again not suits.' However, regardless of what suits they have been achieved in, Phelps admits to being impressed by just how fast people are swimming here. 'There have been some fast swims. The 200m and 400m freestyle really stand out in my mind as being unbelievable records. 'You know Federica (Pellegrini) to go 1min 52sec (at 200M) and Biedermann to go 1min 42sec flat (at 200m). Federica to break 4min in the 400m and then Thorpe's (400m) record to go down (Biedermann beat it). 'There have been some records that have been unbelievable. One of the coolest ones to watch was Cesar's (Celio) last night (in the 100m freestyle). 'That kid is a tough swimmer and he's a best friend in the world so that was cool to see him win.' 