Spurs travelled to West Ham on Wednesday night knowing a victory would have taken them above Leicester, b ut Michail Antonio's first-half strike secured a 1-0 victory for the Hammers and ended Tottenham's six-game winning run.
Having to juggle their Premier League commitments with the Europa League and - until last week - the FA Cup appeared to have taken its toll on Pochettino's players at Upton Park.
The Spurs boss said: "We are near the top after a very busy period. We played Europa League, FA Cup and now three Premier League games in one week. It's tough.
"The Premier League is very competitive, with no easy games. You need to be 100 per cent mentally and physically.
"But there is no excuse, only that we were poor in the first half."
Antonio struck after just seven minutes, heading in Dimitri Payet's corner at the near post.
The closest Spurs came to replying was on the hour mark when Harry Kane failed to connect with the rebound after Adrian saved Toby Alderweireld's long-range shot.
Source: PA