Yamasaki was selected at the number 2 draft pick for the Dragons in 1987. He helped lead the Dragons to the 1999 Japan Series (which they ultimately lost to the Fukuoka Daiei Hawks, 4-games-to-1).
Yamasaki was the MVP of the 2008 All Star Game #1. He had 2 home runs in the 2009Climax Series.
Yamasaki was awarded the 2011 Golden Spirit Award, given to a NPB player who "best exemplifies the game of baseball, sportsmanship, community involvement and the individual's contribution to his team", as voted on by members of the media.
Yamasaki played 24 years in NPB, retiring after the 2013 season with 403 home runs, placing him 17th all-time in Japanese professional baseball.