Spain coach Inaki Saez admitted he could not see his side winning against Russia before bringing on goalscorer Juan Carlos Valeron.
Saez said: "It was not a classic, first games in tournaments never are. We found it increasingly difficult so I had to make changes.
"It's a substitution I use regularly. I needed a player to pass the ball and hold possession. Valeron did that.
"Russia seemed ready for our strikers. They were a good side who worked hard."
Saez now knows, having seen Portugal lose to Greece earlier in the day, that
if Spain can beat the Greeks in their next match, they will be certain of a
He added: "We will have to be at our very best. Everyone saw what Greece did
to Portugal, they may not have any really big names but they are well organised
and we will have to be at our very best to beat them."
Spain goalscorer Valeron praised his team-mates' patience as he secured victory over Russia.
"I thought we were unlucky not to score in the first half but in the end we kept our heads and getting that goal proved to be vital," he said.
"It was quite lucky because it was the first ball I actually touched in the game and it managed to go in.
"We certainly had to suffer a lot to get this victory but in general I thought we played a good game."
Midfielder Xabi Alonso said Spain had been surprised by
Portugal's defeat by Greece.
"It just goes to show that any team can win in this tournament and there are going to be no pushovers," said Alonso.
"We feel a bit easier now that we've won our first match but we're going to have to be very alert for the next game against the Greeks."