The Blues put in a weary-looking performance on Sunday and were convincingly beaten 2-0 at champions Manchester City to leave them looking over the shoulders in the race for a top-four place.
Tottenham can go past Rafael Benitez's side and into third as early as Monday night should they win at West Ham, while Arsenal are just two points further back.
Cahill admits there is reason for concern following a season that started with a buzz of excitement following last May's first Champions League success, but has gradually turned into a slog.
Sunday's game was Chelsea's 46th of a campaign that has seen them travel to Japan for the World Club Cup, while their international players have not had a week's rest since losing the Super Cup to Atletico Madrid on August 31.
By contrast, Arsenal have been the busiest of Chelsea's top-four rivals having played 41 games, followed by Tottenham and Manchester City (both 38) and Everton (32).
"Unfortunately it does take its toll," Cahill told Chelsea TV.
"I think if you look back at the start of the season with the spark and the zip we had - I think sometimes a little bit of fatigue plays its part.
"We haven't had a clear week for about 29 weeks and you're expecting to go into games feeling 100 per cent fresh, trying to push on further in all competitions.
"I think at this stage of the season, it's not an excuse, but fatigue has to play a part.
"They (Manchester City) had a clear week and we've had a ridiculous amount of games.
"I think it told. I thought they were a lot fresher and sharper.
"We talked about it afterwards and we have to keep getting on. There's nothing we can do about it."
There is hardly any respite on the horizon for the Blues either who - despite having already crashed out of five competitions this season, are still alive in three more.
Chelsea are back in action on Wednesday night when they travel to Middlesbrough for their rescheduled FA Cup fifth-round tie before the weekend visit of West Brom is followed by a trip to Romania for the first leg of their Europa League last-16 tie with Steaua Bucharest.
"It's something that we have got to deal with," Cahill said.
"We've got the two cups and obviously trying to get the Champions League spot.
"That's our aim from now until the end of the season. That's our focus. We have to go again. We are still in that position.
"All three are in our own hands so we need to keep going and plug away and make sure we achieve the goals we want to achieve."
Interim boss Rafael Benitez hinted that Sunday's defeat at City had convinced him he must now begin to prioritise competitions.
The Spaniard showed signs of that in midweek, when he left out Frank Lampard and Branislav Ivanovic as Chelsea narrowly avoid a shock exit to Sparta Prague.
Both came back into the starting XI at the Etihad Stadium, along with Eden Hazard after he stepped off the bench to put Chelsea through against Sparta with the last kick.
But even then Cahill believes his side lacked energy as he was left to thwart a number of City attacks before Yaya Toure and Carlos Tevez eventually broke through in the second half.
"I had way too much (to do)," he said.
"Normally at a big club in a game you probably get asked to do three or four important things in a game. Today we probably had 20 or 25.
"Unfortunately if you keep getting asked the question ultimately there's going to be a goal.
"I felt they were on top of us for most of the game. They just kept coming at us and coming at us.
"You defend one and defend two; sooner or later one is going to go in and that's what happened.
"We didn't really create many chances.
"We had to defend as a team a lot and we paid the price in the end."
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