Mata bagged the second goal in Spain's 2-0 win over Georgia on Tuesday as they booked their place in Brazil next summer as Group I winners.
"I am very happy to win, to qualify first mathematically and to go out and score," said the diminutive midfielder.
"In the last qualifiers it has been very difficult to score because it has been tight."
Mata's season began with him spending a lot of time on the bench at Chelsea under Jose Mourinho, but he insists that was because of injury.
"I'm fine, I've been playing several games and I think I am at a good level," he said.
"I feel fine. When I had a difficult time that was in pre-season due to injury."