With Arsenal beating Swansea 2-0 and having a game in hand, the defeat to the Cottagers meant Spurs forfeited their seven-point lead to just four in the race for fourth spot and Champions League qualification.
And Sigurdsson believes the team suffered from exhaustion against Martin Jol's side, following their late victory against Inter in the Europa League after extra time.
"We just never got going," Sigurdsson told reporters.
|5/2||Spurs are 5/2 with BetVictor to win the Europa League
"It was almost as if everyone was a bit down and there wasn't enough energy or movement. There was just something missing from our game.
"I don't want to be making excuses but the 120 minutes from Thursday night [against Inter] probably played a bit of a part [against Fulham], and also the travelling, but we could have played better even if we were tired.
"It could have been a factor but we have to make sure we are fresh and ready for the next game."
The creative midfielder also rejected the idea that Spurs were beginning to falter under the pressure of challenging in Europe and going for the much-coveted fourth spot.
"We are still confident for the Champions League," added Sigurdsson.
"We've shown during the season that we are more than capable. We've had a couple of bad results but we won't start doubting ourselves."