SYDNEY Swans midfielder Kieren Jack readily admits he seriously lacked knowledge of Australian Football when he decided to chase his AFL dream.

The son of rugby league great Garry Jack, he faced some enormous obstacles - and plenty of scepticism - about his potential to reach the highest level in the AFL.

He only made the decision to drop rugby league at the age of 15, but he was determined to make it and will play his 100th AFL match when the Swans take on Essendon at Etihad Stadium on Saturday.

"It's been a tough journey," Jack told reporters ahead of training on Thursday.

"I think I missed a lot of the fundamentals of the game of AFL as a young kid growing up.

"I was a little bit disadvantaged from the Victorian and South Australian kids.

"I just wasn't taught the knowledge of the game as much as they were, which I found out when I got here.

"That was certainly a challenge, but I've had some really good mentors and coaches who have helped me along the way."

Rookie-listed in 2006, Jack made his AFL debut the following season, but only managed two games.

But from 2008 to 2010 he played 69 of a possible 70 games, capping his remarkable journey by winning the club's best and fairest award in 2010.

He faced plenty of doubters along the way, however, including some within his own club, coach John Longmire admits.

"There was a lot of doubts about Kieren Jack initially," Longmire said.

"Everyone thought he was too small to play AFL football and not good enough.

"And considering his dad's background and his heritage with rugby league, it was a challenge for him.

"But he's embraced the game, he's such a strong-minded kid, and he was able to block out all the outside talk and become one of the top-line midfielders."

Asked if he personally had any doubts, Longmire said with a huge grin: "Oh no, I always thought he was going to be a really good player.

"I didn't have any doubts at all and I was the one who said 'We've got to pick him'.

"That's how I recall it anyway."

Garry Jack represented Australia 20 times and NSW on 17 occasions, along with 244 first-grade games with Balmain.

And Kieren said it's fair to say his father's knowledge of AFL is still in the embryonic stage.

"I'd like to say yes, but I still don't think he has [learned the game]," Jack said.

"He still tries to get me to chip over the top of players, so that's where his knowledge has got to.

"He played 244 [first-grade games], so until I get there, he'll still think he has something over me.

"One hundred games is a starting point for him."

James Dampney covers Sydney AFL news for AFL.com.au. Follow him on Twitter: @AFL_JD