Rijker started her martial arts career at the early age of six, when she began training in judo. A year later, she was part of the Dutch National Softball Team. At fourteen, after picking up fencing a year earlier, Lucia became the Netherlands Junior Champion of that sport. When she was fifteen she began kickboxing and knocked out the reigning American kickboxing champion, Lily Rodriguez. Rijker eventually amassed a 36-0 (25 KO) record as a kickboxer, and won four different world titles. Her only defeat in a kickboxing ring was in an exhibition match at Sporthallen Zuid in Amsterdam in October 1994, when she fought male Muay Thai champion Somchai Jaidee, (the then reigning Muay Thai Australasian champion), who knocked her out in the second round.