RETIRING Sydney Swans co-captain Brett Kirk says his early battle to forge an AFL career made the decision to end it even harder.

Kirk, 33, announced on Wednesday that he will call it quits at the end of 2010, bringing a 12-season career to a close.

Unlike many successful players, Kirk’s progression to the top flight wasn’t a smooth one.

He joined the Swans in 1998 as a supplementary-listed player from North Albury, only to be told he wasn’t good enough.

Undeterred, Kirk demanded another chance to prove his worth and it paid off in 1999 when he was elevated from the rookie list after playing five games that year.

Having worked so hard to earn his place as a respected senior player, Kirk admitted it was tough to voluntarily give it away

“I fought against [the decision] a lot, probably because of the way that I’ve got here,” he said.

“I tend to think I was up against a lot and that’s probably what built me, built my character. Challenges, obstacles - I learned how to jump over them, through them, build a bridge to get over them.

“I needed to release it; I’m not great with carrying emotion and I needed to go more with the flow, which I have done over the last five or six years.”

Kirk said a sub-par performance against the Western Bulldogs in round nine, and his subsequent mental torment, was the catalyst for his decision to retire.

“My career hasn’t been built on ability. It’s been built on competitiveness, fight, mental discipline and strength of mind and I think my mental focus was clouded on the weekend,” he said.

“I couldn’t function as I normally do. I want to leave the game playing good football and really trying to achieve something with this group.”

Swans coach Paul Roos said Kirk’s rise from a player on the brink of being delisted to a revered leader of a premiership team left him in awe.

“Brett was really finished as a player in 2002. Stuey Maxfield was really one that pushed his cause and we gave him another opportunity and he played his role, which is a mantra we talk about here,” Roos recalled.

“He was able to play his role for the best part of eight and a half years, and finished in the top three in the best and fairest for seven years and became an extraordinary player.

“I can’t think of a player that’s gone from a position [where] he was effectively out of senior contention, effectively staring down the barrel of having his career terminated at the end of 2002, to becoming an exceptional player and probably in the top five or six players of this football club’s history.”