WBC middleweight world champion Saul Alvarez has announced a surprise showdown with Amir Khan in Las Vegas on 7 May.
Englishman Khan (31-3), who has not fought since a points victory over Chris Algieri last May, said last month he had a "few options" for his next fight, but no one expected him to arrange a catchweight contest at 155lbs with Alvarez.
'Canelo' Alvarez (46-1-1), who is scheduled to meet multiple world title holder Gennady Golovkin in a unification bout later this year, will take on former two-time world champion Khan in his first title defence since claiming the vacant belt with a unanimous decision win over Miguel Cotto in November.
Khan steps up to middleweight after admitting defeat in his quest to face undisputed welterweight kingpin Floyd Mayweather Junior, who he couldn't resist a dig at.
“I know Canelo fights the best and wouldn’t pass up the opportunity to face me like others have,” said Khan.
"My goal is to always fight the biggest names and the best fighters; that is why I’m excited for this fight. I know I have the speed and ability to beat him and will give my fans what they deserve on 7 May.”
Andre Berto, Brandon Rios and Jo Jo Dan had all been tipped as possible warm-up opponents for Khan ahead of a summer showdown with Danny Garcia or Kell Brook, the respective WBC and IBF welterweight champions.
Khan is Garcia's mandatory challenger and the WBC had ordered the American - who had a fourth-round knockout victory over the Bolton fighter in July 2012 - to face him by June or lose his belt.
Mexican champion Alvarez said: “As the middleweight champion of the world, I will take on the best fighters in the sport and on Cinco De Mayo weekend, I look forward to making the first defence of my titles.
"Amir was a decorated amateur, a two-time world champion and is in the prime of his career. Fans are in for a great fight.”
Alvarez's only loss in 48 fights has been a majority points defeat to the unbeaten Mayweather in 2013 and many regard him as the next superstar of boxing when the American decides to hang up his gloves.
Alvarez carries a big punching reputation and has won 46 of his 48 fights - he had a draw against Jorge Juarez in only his fifth contest - by way of 32 knockouts.
Shah Khan, the boxer's father and manager, said his son was relishing the challenge of taking on Alvarez and insisted stepping up to 155lbs from the welterweight mark of 147lbs would not present an issue.
"Amir is at a level now where he's mixing with the big guys and big names, he's up for this fight and he deserves it," Shah Khan told Sky Sports.
"When he was younger and went to the Olympic Games people said: 'He can't do this, he can't handle these fights'.
"But look what he did at the Olympics (Khan won a silver medal as a 17-year-old) and he's done exactly the same as a professional.
"It's eight pounds difference in his weight but Amir is a big guy as well. Amir's got a good reach, he's got a good physique and he's made for that sort of weight.
"He will build up his weight well and Virgil (Hunter, Khan's trainer) will put muscle on him and build him the right way."
Khan senior insisted the big domestic fight with Brook could happen at a later date after negotiations between the two camps broke down.
"The fight's always there, it can happen," Khan said. "We were in negotiations with them and Eddie gave Amir some numbers, but he just couldn't deliver them."
