Chelsea striker Timo Werner was unable to take a penalty against Tottenham because of cramp, manager Frank Lampard confirmed.
After both sides scored nine combined penalties in the shoot-out, Mason Mount missed the 10th penalty to book Tottenham's place in the Carabao Cup quarter-finals.
Werner, who scored in regular time, was not one of Chelsea's five spot-kick takers.
Speaking after the game, Lampard said: "We looked tired at the end which was understandable given where we are at. Timo Werner had some cramping at the end so he didn't take a penalty."
The Englishman added that he wanted to see his side be more positive after going ahead through Werner.
"Anything can happen on penalties, you want to stay in these competitions of course but there's lots to like. We dominated the first half but the game changed, I'm happy with bits.
"They went long in the second half, missed our press out, a clear change in tactics from them and we need to improve on that. We can react better. If we get the second goal when on top and be more positive - that will come."
