Ritchie (48), New Zealand's most prolific classical composer, won 13 of his 15 games in the round robin competition at the Te Mata club to win the trophy from William Rush (Nelson) 12 wins, Sue Lea (Wellington) 11, and Pam Fisher (Auckland) 10.
The crunch game was against Lea in round 13 during which both shared the lead.
Ritchie won that game 26-22.
It was a remarkable performance by the Punga club player who is now ranked 11th in the country and has a world ranking in the top 100.
Players are invited to play in the Edwina Thompson competition if they are ranked between 11th and 26th in New Zealand.
"My form going into the tournament was not that good," Ritchie said.
"But the conditions were good and I had a good start."
Ritchie, who made his international debut for New Zealand A in Australia in 2005, has now won five national events.
He also represented New Zealand A against Australia in 2007.
A lecturer in music at the University of Otago, Ritchie is the best croquet player in the province.
"I must juggle my time and can only play in two major tournaments each year," he said.
"I like the game. It is like chess on grass and combines the mental and physical skills. It is something completely different for me."
The highlight of the tournament for Ritchie was to make 17 points from a triple peel.
"It is like making a century in test cricket," he said.
"It is an endurance test and requires a lot of concentration."
