Penn's book, Mixed Martial Arts: The Book of Knowledge, in helping him learn during his early years in MMA. Riddle then fought Dan Simmler, an understudy of Matt Serra, to get into The Ultimate Fighter (TUF) house. Riddle knocked out Simmler in the opening seconds of the second round and struck him three times in the jaw after losing consciousness, breaking Simmler's jaw into two places. Riddle was picked second overall for Team Rampage. During his involvement on show, Riddle was given the nickname Chipper after Rampage noticed him always smiling. Riddle's second match was against Tim Credeur. Credeur defeated Riddle in the second round, causing him to be knocked out of the tournament. After the fight, Credeur bought Riddle an Xbox 360 with his earnings. Ultimate Fighting Championship (2008–2013) At The Ultimate Fighter 7 Finale, which was held on June 21, 2008, Riddle made his UFC and professional debut defeating fellow Ultimate Fighter castmate, Dante Rivera, via unanimous decision (29–28, 30–27, 30–27). During The Ultimate Fighter 7, Riddle was shown multiple times arguing with the older and more experienced Rivera. Dante, on more than one occasion, stated that he would retire if Riddle defeated him. Riddle was then scheduled to fight Ryan Thomas at UFC 91, but a knee injury forced Riddle out of the fight and got pinned. The knee injury's recovery was scheduled to take about 6–8 weeks. After his recovery, Riddle fought and defeated Steve Bruno in a unanimous decision (29–28, 29–28, 29–28) at UFC Fight Night: Lauzon vs. Stephens. In his third bout, Riddle defeated another one of his former Ultimate Fighter 7 castmates, Dan Cramer, via a unanimous decision on the undercard of UFC 101. He next faced The Ultimate Fighter 9 alumni Nick Osipczak on November 14, 2009, at UFC 105. The fight was held in Osipczak's home country of England. Riddle lost the fight by TKO in the third round. After his loss to Osipczak, Riddle faced newcomer Greg Soto on March 27, 2010, at UFC 111. After a dominant first two rounds for Riddle, Soto landed an illegal upkick to Riddle's head in the third round which rendered him unable to continue, giving Riddle the victory by disqualification. Riddle faced DaMarques Johnson on August 1, 2010, at UFC on Versus 2. The fight was a catchweight bout after Johnson weighed in at 172 lbs and Riddle would go on to win