King's Cup holders Valencia have had no trouble scoring this month but will have to stop leaking so many goals if they are to get past 2007 winners Sevilla and reach the semi-finals.
With their attacking line including Spain internationals David Villa, David Silva, Juan Mata and Joaquin on sparkling form, Valencia have scored 12 goals in their five matches this year but have won only two of those games.
Their attempts to plug the gaps at the back will be made harder by the absence of keeper Renan after the Brazilian was injured in Sunday's 3-2 defeat at Athletic Bilbao and could be out for several weeks.
The quarter-final first leg is in Valencia on Wednesday with the second leg in Seville scheduled for next week.
"We just haven't been able to kill games off," Villa told Valencia's website on Monday.
Sevilla are third in the Primera Liga with Valencia one place below. Both sides are desperate to win lucrative places in the Champions League next season but Villa and Sevilla goalkeeper Andres Palop insisted the Cup was not an unwelcome distraction from that task.
"The competition is very important," Villa said.
Palop added: "We are very close to reaching the final stages of the competition so of course we will be focusing all our attention and energy on the match.
"We are not going to give priority to anything, just take each game at a time."
Wednesday's other first leg tie pits struggling Espanyol against city rivals and runaway league leaders Barcelona (2000 GMT), while on Thursday, improving Athletic Bilbao host Sporting Gijon and Real Betis play at Real Mallorca (both 2000 GMT).
The Barcelona derby brings two teams together who are experiencing wildly contrasting fortunes.
Barca broke the record for the number of points amassed in the first half of the season, with 50 from 19 matches, while Espanyol have not won in the league since Nov. 2 and are mired in the relegation zone.
Barca defender Daniel Alves said it would nonetheless be a difficult match as derby encounters were always different.
"We will very much have to be on our guard as they are not going to hand us anything on a plate," the Brazilian international told the club's website (www.fcbarcelona.com). Espanyol's Daniel Jarque said the clash with Barca was coming at a difficult time for the club.
"In any case, a win would give us a lift," the defender said on Espanyol's website (www.rcdespanyol.com). "You always have to find the positive in every situation."