The semi-final second leg at Stamford Bridge on Thursday night ended 1-1 – and 2-2 on aggregate, meaning a penalty shootout was needed to separate the two teams.
However, despite Cesar Azpilicueta seeing his spot-kick saved by Kevin Trapp, Kepa Arrizabalaga then kept out two Frankfurt penalties to be his side’s hero in the shootout.
“There is no preparation for penalties because you can kick very well in the training ground, but then the pressure is different,” Sarri told BT Sport.
“You need a great personality, a great character. I think it’s really useless to try [to prepare for] penalties.”
“Now we need to rest. We have to recover energy because we are really very tired.”
As for the match itself, the home team had made an excellent start after taking a 28th-minute lead thanks to Ruben Loftus-Cheek’s well-taken goal.
However, the visitors responded through Luka Jovic’s 49th-minute effort and then took the game to their opponents, twice forcing dramatic goal-line clearances in extra time.
“I think we played a very good first half,” said Sarri. “Then in the first part of the second half, we went in trouble. We were passive in the defensive phase, so we conceded the goal, 10 minutes of panic.
“Better in the last part of the match, then in the extra-time, I think we were really tired – after 60 matches to play 120 minutes it’s really very difficult and we were unlucky during the match because we lost (Andreas) Christensen, we lost (Ruben) Loftus-Cheek.
“We started the match with three injuries and two injuries during the match, it’s not easy but in the end, we can play the final.”