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 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. "Unfortunately it does take its toll," Cahill told Chelsea TV.
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).
"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," Cahill added. "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."
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