Our chances against Real? Three percent - Sarri

"It's an almost impossible game for us because they score all the time," Sarri told media at Napoli's training ground a day ahead of Tuesday's last-16 clash at the San Paolo, estimating his side's chances of success "three percent".

If Napoli are to go through on away goals, the Serie A side will have to do what no team has managed since Wolfsburg beat Madrid in a quarter-final first leg nearly a year and 46 games ago -- keep a clean sheet against the reigning Champions League winners.

Napoli, looking to reach the quarter-finals for the first time, have been playing arguably the best football in Italy this season, thanks in large part to on-form Belgium midfielder Dries Mertens. 

During a four-month injury absence for Napoli's Polish international striker Arkadiusz Milik, Mertens was pushed up front and has responded by taking his league tally to 18 goals.

Mertens is expected to start up front, flanked by Lorenzo Insigne and former Madrid forward Jose Callejon, in Sarri's preferred 4-3-3 formation.

Although Sarri expects Napoli to do damage up front, he knows causing an upset will require a more combative approach and for the fans to do their bit too.

"They're the favourites but we can make life difficult for them," said Sarri, who led Napoli to the Serie A runners-up spot behind Juventus last season.

"For me, the pressure's on them. They're the richest in the world, the European champions and the world champions.

"Our world champions are our supporters and tomorrow all of Europe will see that."

But Sarri warned: "With the physical power and technical ability they have it's going to be difficult. Our defensive game will be crucial for us, as well as paying attention and making sacrifices."

With Napoli looking for goalkeeper Pepe Reina to keep a clean sheet, Sarri added: "The biggest challenge will be trying to contain them for as long as possible.

"In attack I think we'll have our chances. But we go into this game two goals down. In simple terms, we have about a three percent chance of success, but it's better than nothing."

Asked whether Napoli would consider man-marking Madrid, Sarri quipped: "I thought about sending out 12 players, but they told me I couldn't."