Following their disappointing defeat to Manchester City on Sunday, the Blues have played 46 games in seven different competitions this season, including four matches in the last 10 days
That figure is more than any of Chelsea's Premier League rivals and Cahill has claimed that City benefited from their less demanding schedule at the Etihad Stadium.
"Unfortunately, it does take its toll," he told ChelseaTV. "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.
"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."