Former Spurs player Dimitar Berbatov scored the winner as Fulham beat Spurs 1-0 at White Hart Lane on Sunday, while Liverpool secured a 3-2 victory at Anfield on 10 March.
Those defeats came either side of a 4-1 loss away to Inter Milan in the Europa League, although Andre Vilas-Boas' side still advanced on the away goals rule after the tie finished 4-4 on aggregate.
Tottenham's north London rivals Arsenal are now just four points behind them in the league but Iceland international Sigurdsson is confident of bouncing back in the final eight games of the league campaign.
"We just need to pick ourselves up, stay together and perform well until the end of the season," he said.
"It (a top-four finish) is in our hands and we are in a good position to make sure we get the Champions League spot. We showed during the season that we are more than capable.
"Even though (we've had) a couple of bad results, we won't start doubting ourselves.
"It's down to us now and we have just got to make sure we are playing the way we've been playing throughout the season."
The Fulham game came just three days after the night game in Milan and Sigurdsson feels fatigue may have been a factor in their lacklustre performance.
"I don't want to be making excuses, but the 120 minutes probably played a bit of a part from Thursday night and also the travelling, but we could have played better even if we were tired," he said.
"It could have been a factor, but we have to make sure we are fresh and ready for the next game.
"The lads can pick themselves up. We have got the determination to get into the Champions League.
"We showed at Arsenal at home that we can beat anyone here so there will be a good few games coming up and hopefully we can get a few points."