Spain, who have yet to concede a goal at the tournament, produced a dominant display as Mikel Oyarzabal scored twice and Pedro Porro once to underline their status as one of the favourites.
"I have many reasons to be happy today. First, because of the victory, we won significantly, and the image of the team was fabulous," De la Fuente told reporters. "But there is still room for improvement. Perhaps you think you have seen the best of us, but this team has not yet reached its capabilities. There is more to do."
De la Fuente said Spain's progress had been built on a long-term process rather than one standout performance and stressed that the knockout rounds would demand even higher standards.
"Today's performance should be the foundation for what comes next," he said. "We are going to keep improving and doing things better... We are not yet satisfied - we are going for more."
De la Fuente cautioned against believing Spain had reached their peak after another impressive defensive display in which Austria failed to register a shot on target.
"There were situations where we lacked high pressure. You need to keep improving all the time," he said. "When you get flattered and believe it, you become weaker. Satisfaction can kill you."
The 65-year-old was full of praise for his left back Marc Cucurella, who provided two assists for Oyarzabal, calling him "invaluable" and "a top-notch player".
"The next phase will be increasingly demanding," De la Fuente added. "We have wonderful players, but the only focus is to exceed our own expectations."
Read the match report with Flashscore.
FIFA World Cup 2026
The 2026 World Cup is taking place from June 11th to July 19th in the United States, Canada and Mexico. The tournament features 48 national teams and is played at 16 modern stadiums.
Match schedule and times | Group tables | How to watch the World Cup | World Cup Format | Past winners of the World Cup