Experienced Spain defender Cesar Azpilicueta said on Saturday he was excited by the squad's "bold" youngsters as La Roja set their sights on a second World Cup crown.
The 33-year-old Chelsea captain, one of the few veterans in Qatar with Spain, said he enjoys watching them play and is impressed by how rapidly they have adapted to top-level football.
"What surprised me most is their boldness, they're not afraid – the way they fit into the game, it's a pleasure to see this readiness for the age that they have," Azpilicueta said on Saturday.
"In football the youngsters start earlier and earlier. They are mature, they understand the game and they are showing it. Some of them have played in the Euros already, in the Nations League. At 18 years old they are at a World Cup."
Another youngster linking up with the squad is 19-year-old Barcelona defender Alejandro Balde, who will land in Doha on Saturday afternoon to replace injured left-back Jose Gaya.
Azpilicueta, who made his Spain debut in 2013 under World Cup-winning manager Vicente del Bosque in a friendly in Doha, believes Balde is ready for the tournament despite being uncapped.
"We offer our support to Jose," he said. "Yesterday was a sad day for all of us and now we will try and give our all for him, because he’s a big loss on and off the pitch.
"Now a great teammate is coming, he's shown his quality with Barca, and with the under-21 side, he’s physical, is very fast and the group will welcome him from the first minute."
The 2010 World Cup winners trained on Saturday under Luis Enrique's watchful eye, with Marco Asensio returning after missing the Friday session.
Azpilicueta hopes to be able to make more of an impact for Spain in his third World Cup, after failing to get off the bench in Russia four years ago.
"While it's true I haven't had consistent game time, I've never been able to play at the level I have with Chelsea in the national team," added the right-back.
"You have to be self-critical and realistic, and for that reason I've had a little thorn in my side."