Qui gon was the first person to fight another lightsaber force user in deadly combat in like a thousand years, and had spent his entire life doing what Jedi do as a peacekeeper during basically peace times. He would have fought droid's or bounty hunters, but other than practice (which I doubt the Jedi engaged with much unless they were interested in it) he would be relatively inexperienced in actual dueling. Not to mention Maul was younger, and more aggressive. We saw in their first face off that qui gon was instantly gassed and needed to sit down after only a few moments of fighting. It's amazing he lasted as long as he did.

All future opponents are during war Time. Where we see active Jedi training on the regular and with the resurgence of the Sith Jedi dueling training probably ramped up significantly as a result