He has previously held the German BDB cruiserweight title and the EBU-European Union (European Union) cruiserweight title. He boxed his first professional fight in San Jose, California, winning the bout against Fred Simmons, by unanimous decision. This was however followed by his first professional defeat at the hands of Turkish boxer Turan Bagci in Spain.
After 2 successful title defenses against Michael Klötzsch and Firat Arslan, May was given a shot at the EBU (European) cruiserweight title in a bout against Ukraine"s Alexander Gurov in 2001.
Rudiger lost the bout however by TKO in the 4th round, his second pro defeat. In 2002 he boxed Alexander Petkovic, with both the WBC International cruiserweight title and German International cruiserweight title at stake.
The fight ended in a disappointing draw. He boxed to another draw and the belt remained with Cantatore.
That same year May"s promoter Wilfried Sauerland secured him a world championship fight with WBO title holder Johnny Nelson.
May missed his opportunity though, losing the bout by TKO in the 7th round. In April 2005 May fought Poland"s Krzysztof Wlodarczyk for the vacated EBU-European Union (European Union) cruiserweight title but was beaten by knockout in the tenth round. Due to an alleged lack of prospects in professional boxing, May"s promoter Sauerland arranged a better matched duel for May against the emerging and then still undefeated Marco Huck.
May lost the fight on points over ten rounds.
After a two year break from boxing, May returned to the ring on 31 March 2007 in a successful comeback fight against Sven Hazel. After two more victories he was handed another world championship fight, this time against WBC International cruiserweight title holder Herbie Hide in March 2008.
May lost the bout by technical knockout in the second round, Hide"s power proving too much for May, despite his height advantage.