Despite launching from the hill significantly lower down than the other jumpers, and taking off at the slowest speed of the round, the 28-year-old recorded a leap of 143 metres - the furthest of the day.
"To do the longest ... is good for me. I try of course to impress the other guys, (and) to go on in that fabulous form I'm in. Of course, it's much easier to enjoy it after 143 metres," he said.
Ammann, who won both the normal and large hill contests at the 2002 Games, easily took the normal hill title last week.
He will become the first man to capture four individual jumping golds if he wins Saturday's large hill contest.
"I think it's important to have stable form. That's important for the mental approach to the jumping," he said.
Ammann almost fell on landing and said the conditions at the bottom of the slope were too sticky.
"The take-off was quite good but I think still there is still some small work to do. But I think I will keep my good shape," he said.
Ammann's biggest rivals on paper will be the strong Austrian team, which performed disappointingly on the normal hill and could only win a bronze.
"I would not underestimate the chance or the tasks on the big hill but the chances are getting fewer for the other guys," Ammann said with a smile.
In their defence, the Austrians are much more used to the large hill.
Gregor Schlierenzauer, last year's World Cup winner and third in the normal hill behind Ammann, is an expert glider while teammate Thomas Morgenstern won the large hill in the 2006 Games.