Early career

In the 2005–06 season, Kozuka won the Junior Grand Prix Final, the Japanese Junior Championships, and the World Junior Championships.

2006–07 to 2008–09

Kozuka debuted on the ISU Grand Prix of Figure Skating series in the 2006–07 season. He won the bronze medal at the 2006 NHK Trophy and placed 6th in the 2006 Trophée Eric Bompard. He placed 6th in the 2006–07 Japanese Championships and 4th in the 2007 Asian Winter Games.

In the 2007–08 season, he won the silver medal at the 2007–08 Japan Championships. He placed 8th at the 2008 Four Continents and 8th at the 2008 Worlds.

In the 2008–09 season, Kozuka won gold at the 2008 Skate America and silver at the 2008 Trophee Eric Bompard.^[12] He qualified for the Grand Prix Final where he won the silver medal. Kozuka won another national silver medal at the 2008–09 Japan Championships. He won the bronze medal at the 2009 Four Continents and placed 6th at the 2009 Worlds.

2009–10 season

In the 2009–10 season, Kozuka won the silver medal at the 2009 Cup of Russia earning 215.13 points and placed seventh in the 2009 NHK Trophy scoring 186.00. He won the bronze medal at the Japanese National Championships with 236.13 points, placing second in the short program and third in the free skate, and thus qualified to compete at the 2010 Winter Olympics and at the 2010 World Championships.

Kozuka finished eighth overall in the men's singles at the 2010 Winter Olympics with 231.19 points.^[13] At the 2010 Worlds he finished in tenth position with a total of 216.73 points.

2010–11 season

Kozuka's assigned events for the 2010–11 ISU Grand Prix season were the 2010 Cup of China and the 2010 Trophée Eric Bompard. He won the 2010 Cup of China with 233.51 points, placing first in both the short program and the free skate. He went on to win the 2010 Trophée Eric Bompard with 248.07, again winning both segments of the competition.^[14] He was the top qualifier for the men's event at the 2010–11 Grand Prix Final.

During a practice session at the Grand Prix Final, Kozuka inadvertently collided with Daisuke Takahashi.^[15] Both were shaken but went on to compete at the event. Kozuka said, "I apologized to him and he accepted the apology with a smile and claimed that he was not in pain but I still feel awful about it. I promised him and his coach that it will never happen again."^[16] Kozuka was fourth in the short program and second in the free skate, and won the bronze medal. He won his first national title in December 2010, placing first in both the short program and the free skate to total 251.93 points. At the 2011 World Championships, he placed sixth in the short program but moved to second overall after the long program and won his first World medal.

2011–present

Kozuka was assigned to 2011 Skate America and 2011 NHK Trophy for the 2011–12 Grand Prix season, and he won the bronze medal at Skate America and the silver medal at NHK Trophy. Kozuka earned the silver medal at the 2011 Japanese national figure skating championships,^[17] thus qualifying for a spot on the Japanese team for the 2012 World Championships, where he finished 11th.

Kozuka began the 2012–13 season by winning gold at the 2012 Skate America and then won silver at the 2012 Rostelecom Cup. He injured his right instep in mid-December 2012.^[18] Kozuka finished 5th at the Japan Championships.

In 2013–14, Kozuka earned a bronze medal at the 2013 Cup of China. He placed 3rd at the 2013 Japanese National Championships but was not included in the Japanese team to the Olympics. He was assigned instead to the 2014 Four Continents Championships where he won the silver medal. When Daisuke Takahashi withdrew from the 2014 World Championships, Kozuka replaced him as a substitute and placed 6th at that competition.

In 2014–15, Kozuka placed 8th and 6th, respectively, at Skate Canada and Rostelecom Cup.^[19] He placed 6th in the short program at the 2014 Japanese National Championships, but rallied with a second-place free skate to place 3rd overall. He was assigned to the 2015 World Championships, where he finished 12th.

Toward the end of his career, Kozuka had tendinitis in his left ankle.^[3] He announced his retirement from skating on March 15, 2016.^[6] On April 17, he performed his farewell exhibition program, Epilogue, at the last show of 2016 Stars on Ice Japan in Tokyo.^[3] In 2017, he said that he would remain involved in figure skating.^[20] He started the Kozuka Skate Academy in 2016 and also works as a skating commentator for Fuji TV.^[21]

Senior results

Junior results