TOKYO -- Bulgarian-born sumo wrestler Kotooshu, the first European to win the prestigious Emperor's Cup, has announced his retirement.

The 31-year-old Kotooshu, whose real name is Karoyan Ando, was promoted to sumo's second-highest rank of ozeki in 2005 and won the Emperor's Cup in 2008 for his only championship at the elite level.

"I was at my limit physically and mentally," Kotooshu said. "I've had so many injuries recently, but I have no regrets because I gave it my utmost over 12 years. I am glad that I was able to be a sumo wrestler."

He was relegated to the third-highest rank of sekiwake for this year's New Year tournament in January where he finished with a losing record and was unable to regain his ozeki status. Battling a series of injuries, Kotooshu lost nine straight bouts in the ongoing spring tournament.

A Japanese citizen, Kotooshu said Thursday he plans to instruct junior wrestlers as an elder of Japan Sumo Association. He ends his career with a record of 537 wins and 337 losses.