Troy Deeney scored the only goal of the game to give the Hornets three precious points the Championship promotion race.
"This was a six-pointer," Zola told reporters. "It was even more difficult than the other ones.
"This team never lets me down. Every time there's a game which is very important, they always perform. That shows you the character and temperament of this group."
Watford are third in the table now, only a point behind second-placed Hull.