Roberto is anticipating a difficult tie against Manchester United but believes Olympiakos can rise to the challenge and reach the quarter-finals of the UEFA Champions League.
The Olympiakos goalkeeper is not paying attention to United's poor record this season, although he accepts that they are under pressure to perform.
He does not believe there is anything to choose between the teams and has challenged Olympiakos not to pass up such an opportunity to reach the last eight.
The first leg takes place in Greece on Tuesday and Spanish 28-year-old Roberto hopes the intimidating home atmosphere will help them take control of the tie.
He told Sky Sports: "Manchester United are not having the best time this season but they are always a big team. We know it will be a hard round.
"For me now, the chances are 50 per cent. History does not influence these matches. We have an opportunity to enter the quarter-finals and this is our challenge.
"Olympiakos are strong at home and with the atmosphere, we have a little advantage against United. But the key is to do it on the pitch.
"We have analysed how to play the English side over the past few weeks and the main thing is to stop the midfield and make the most of our chances. Any goal could be decisive in this round.
"To play against United is a dream for the players and we want a positive memory of this round.
"The pressure is on them but we also have to give 100 per cent from the first minute and we are confident we have the spirit to triumph."