The former Chile manager read out his team in an unexpected move, but explained he had made up his mind and saw no point in concealing it further.
Marcos Rojo and Maxi Meza were the only question marks, but have been given the nod to start at Spartak Stadium against the UEFA EURO 2016 quarter-finalist.
Asked if he had revealed his team early because he did not think Iceland was particularly strong, Sampaoli responded: "No, no, no, the reason why we have named our team in advance is because we knew since Wednesday what we wanted to do.
"I've told you today because we have trained with the starting XI and I don't believe it's necessary to hide this information.
"We know Iceland's main man, their captain [Aron Gunnarsson], is recovering from injury but they play well as a group and I'm sure it's going to be very tough against them.
"They defend in depth and will probably play on the counter attack. Iceland will be a tough team to beat."
On explaining the selections of Meza and Rojo, he added: "Marcos has been a possibility from the beginning, he gives us many options. He hasn't played a lot over the last three months [with Manchester United] but we decided he was ready.
"Meza is the person we have been looking for, he's very versatile and allows us to recover balls. He is ready and just as good as the players he plays with."
Having fended off questions about why Lionel Messi had not fronted up at the press conference – Ajax left-back Nicolas Tagliafico was chosen instead – Sampaoli heaped praise on the Barcelona superstar and claimed players of his calibre changed a coach's thinking.
"It is true that the coach organises the team but the footballers create the opportunities on the pitch and if you have players like Messi then the coach has to organise players around him to take advantage of his amazing skills," he said.
"He supports the team and the team supports him. Having a player like Leo means you have to organise the team differently."
There has been talk of this being 30 year-old Messi's last World Cup, but Sampaoli does not believe that places him under any further pressure.
"He's very fit, well prepared and looking forward to this World Cup and he hopes to be able to achieve his dream," Sampaoli said. "He's a player who is very skilled so I don't think he's under pressure, he shouldn't be under pressure, he is a player who makes people very happy with his game.
"I don't think this should be Messi's last World Cup. His skills and the way he plays mean that he will be the one who decides.
"He is a genius and he plays like a genius, I don't think it will be his last."