Spurs salvaged a 2-2 draw in the first leg at White Hart Lane after going 2-0 down against the Swiss champions.
"We need to go out there and win the game," Sigurdsson told the club's official website. "We know what we've got to do.
"We've got to attack them and win the game, whether that's in the first minute or the last. The important thing is to be in the hat for the next round."
Sigurdsson scored Tottenham's equaliser in the last leg to take his tally to six goals for the season.