"Tomorrow we face the world champions who consistently demonstrate their prowess in how they play football," Williamson told reporters at St Jakob-Park in Basel on the eve of the match.
"They are an exceptional football team, the best in their field, and we are fully aware of that, but we believe we excel in certain areas as well.
"We must perform at our highest level to overcome Spain, but I believe they will also need to be at their best to defeat us.
"I think there is mutual respect between the two teams, and I would hesitate to label us as underdogs in this situation."
England aims to defend the European Championship title they secured on home ground in 2022, but the match is also a rematch of the 2023 World Cup final, where Spain emerged victorious 1-0 in Sydney.
A fierce rivalry is developing between the countries, with England winning in extra time during their encounter in the quarter-finals at Euro 2022.
The teams have already met twice this year in the UEFA Nations League, with England winning 1-0 at Wembley in February.
Spain avenged that defeat by winning 2-1 in Barcelona at the start of last month, when Claudia Pina scored twice after Alessia Russo had put England ahead.
"We have faced each other frequently over the past few years, and I believe there are always lessons to learn from those matches," said England coach Sarina Wiegman.
"Tomorrow presents a new opportunity where we can hopefully neutralize their strengths and exploit some of their weaknesses as well."
England has been concerned about the fitness of Lauren James for the final after she sustained an ankle injury during the semi-final victory over Italy, but Wiegman suggested that the winger is likely to play on Sunday.
"We had 23 players on the pitch today and everyone came through," said the Dutch coach.
Chloe Kelly and young striker Michelle Agyemang are vying for starting positions after making significant contributions off the bench in comeback victories against both Sweden and Italy in the last two rounds. However, Williamson believes James has a lot to contribute.
"We have a squad full of talented players. Lauren brings something unique, and having her available is the optimal scenario," said the Arsenal center-back.