Portugal are obviously the heavy favourites, but this will be probably be a very tough game for them. As Slovenia showed in the group phase, they are a very difficult team to play and they are yet to lose a single match at these Euros.
Vitinha and Palinha start in central midfield for Portugal, they look a strong partnership to me. While in front of them Fernandes will be responsible for the play-making duties and he will be flanked by Bernardo Silva and Leao. Both Pepe and Cristiano Ronaldo start again for Portugal, at 41 and 39 years of age, it's quite incredible how their still playing well at this top international level. Dias will partner Pepe in central defence and Dias has been pretty solid in this competition so far.
For Slovenia, they are a strong side defensively and they can play a bit when they attack as well. When you have world class players like Oblak as goalie and Sesko in attack, you are always in with a chance of winning even against top opposition.
Portugal are strong favourites against Slovenia, but this will likely be a tricky match for them.