After going two goals behind to Southampton at White Hart Lane on Sunday, Christian Eriksen's double looked to have rescued a draw for Spurs before the Icelander struck the winner in injury time.
The victory puts Tim Sherwood's men five points behind fourth-placed Arsenal, although Arsene Wenger's side have played one fewer game.
"I think it's going to be very difficult. It's out of our hands now," Sigurdsson told talkSPORT. "The most important thing for us is to concentrate on one game at a time. We've got Liverpool next, and that's going to be very difficult with the way they are playing.
"If we go on a little run, we'll see what happens at the end of the season."