Spurs manager Andre-Villas Boas says his side ran out of energy as they gave up a one-goal lead on Panathinaikos in the second half of their match in Athens.
"We felt the strain physically in the second half and we lost control of the game," Villas-Boas told ITV's Peter Drury. "It's a pity because in the first half it looked like we had the game under control."
The Portuguese also cited the game-changing half-time substitution for Panathinaikos, who introduced the dangerous Ibrahim Sissokho into the fray.
"The number three came on for them Sissokho really changed the game," Villas-Boas said. "He was very creative and created lots of problems for us."