But the 29-year-old produced an impressive display against Japan and, with Adam Jones struggling with a foot injury, he is now favourite to retain the jersey for next weekend's showdown with Fiji.
"When I do get my chance with Wales it's up to me to put my marker down," said Horsman.
"Every time I have been given that opportunity - a few years ago in the autumn, this year's Six Nations and now - I keep staking my claim.
"I'm getting to the point now where I've played 13 internationals and I want to cement my place in the team.
"Adam and I get on really, really well. Generally, in my position I don't normally get on that well with the people I'm competing with - but Adam is a great guy.
"We've realised in a World Cup we are not going to play all the games and whoever gets the job the other one roots for him. But we are also trying to put pressure on each other."
Jones has been on crutches since last Monday after injuring his foot but remains hopeful he will be in a position to compete with Horsman for that number three jersey.
"I think I've torn a muscle and the bleeding has gone down into the nerve. I'm not an expert but it's a bit sore. Touch wood I'll be able to play against Fiji," said Jones.
Horsman views such setbacks differently to most after successfully fighting two bouts of cancer.
Gareth Jenkins' decision to leave him out entirely for the Australia game hit Horsman hard but he has had worse and actually left the meeting with a new-found respect for the coaching staff.
Jenkins may have come in for fierce criticism from some of the local media in Wales, but he has Horsman's undivided support.
"I was bitterly disappointed to miss the Australia game. When I got told by Gareth and Robin McBryde I was absolutely devastated and really cross," said Horsman.
"But that's the type of reaction they wanted."