The rematch will have to wait.

Strikeforce lightweight champion Josh Thomson has withdrawn from his first title defense against Gilbert Melendez at Strikeforce “Shamrock vs. Diaz” on April 11 at the HP Pavilion in San Jose, Calif., according to the promotion.

Thomson (16-2) broke his leg just above the ankle on Wednesday in final sparring preparations for the bout and will undergo surgery on Friday. A search is underway to find a late replacement to face Melendez.

Thomson’s five-round shutout decision over Melendez last June is considered one of the high points of the San Jose fighter’s career, as he handed the well-regarded Pride Bushido star only the second loss of his career.

Melendez (14-2) hasn’t stepped back into the cage since the defeat. The voracious 26-year-old wrestler holds key victories over UFC standout Clay Guida and former Pride star Tatsuya Kawajiri.

In Strikeforce’s main event, former UFC middleweight champion Frank Shamrock will take on the equally outspoken Nick Diaz at a 179-pound catch-weight. Bangers Benji Radach and Scott Smith will duke it out in a featured middleweight bout.

Strikeforce “Shamrock vs. Diaz” will air live on Showtime.