The book,"The Secret Realm of the Final Battle of Online Games", might match the information you provided. The male lead was a former substitute player who had achieved very good results in a closed beta game called "Secret Realm." However, due to academic reasons, he had given up on his eSports career. The male protagonist returned to the game and challenged 100 legions around the world in a 10,000-man competition. Not only was his competitive strength superb, but he also had a wealth of gaming experience. However, the male protagonist forgot his account number and needed to start over. I hope my answer can help you. Muah ~