Portugal face their neighbours and current European champions Spain with Germany facing Italy the following day.
Gullit, writing on Laureus.com, says: "I'm really pleased at how Ronaldo has grown during the tournament.
"It's always good to see a star rising to expectation and he is on fire at the moment. He has a great eye for goal and pace and he knows the Spanish defenders. If there is one moment where he gets the ball, he is quick and he could win the match for Portugal."
Gullit played 66 times for Holland and lifted the European Championship trophy with Holland in 1988, beating England during the Group stages of that competition.
But he sympathised with England who went out of yet another major tournament on penalties after drawing 0-0 with Italy.
He added: "England have an unbelievable curse to have lost again on penalties. I have no idea why they do so badly but they have lost so many shoot outs in big tournaments and to lose on penalties again is heartbreaking for them.
"It doesn't matter how much you practice, that walk from the centre circle to the penalty spot, with the eyes of the world watching you and all the pressure is something you just cannot train for."
Gullit, who played for AC Milan and Sampdoria in Italy and played for and coached Chelsea and Newcastle in England, is undecided about the winners of the second semi-final.
He added: "Germany have certainly been the best team to watch and they must be favourites. They have a good balance of players and are a strong team but I have liked Italy as well.
"I was very pleased and surprised how Italy have played, even though England gave Pirlo far too much time. Italy also have good strikers and they're a good tournament team. It could go either way."