The Red Bull driver had been six tenths of a second clear of his rivals following his first pole shot.
"It is a weird feeling because you stand in the garage when there are two minutes left and know it is too late and see others on final attempts and there's nothing you can do," said the world champion.
"I was watching the sector times very closely. Mark started with purple sector one, Romain had a purple sector two but fortunately my last one strong enough to stay ahead. It was close with Nico.
"It was a great feeling because it could have gone wrong.
"Today is more what we expected, especially with Mercedes being very close behind."
Vettel admitted he might have misjudged how much a second run would be worth.
"I knew we would have a cushion which is why we decided not to run again," he said.
"What we don't know is how much the track improves, plus the fact that if you run again you get more feel for the tyres and track.
"We thought it was good enough, in the end it was."