Mendy was making his first start for the club since his £23m arrival from Rennes and the Senegal international produced a good save in the second half to keep out Sergio Reguillon.
The Blues took the lead in the first half when Timo Werner scored his first goal for the club following a £53million move from Red Bull Leipzig but Erik Lamela levelled with just six minutes remaining to take the game to penalties.
The 28-year-old was signed as a safer pair of hands in contrast to Spaniard Kepa Arrizabalaga but Mendy looked shaky on crosses and failed to save any of the five spot kicks he faced.
However, Lampard said he was pleased with Mendy’s overall performance and said he took good command of his box.
‘I thought he was very good,’ said Lampard.
‘He made a couple of good saves. Came and caught a few balls. Very very pleased with every part of his game.’
Chelsea’s superiority showed in the early stages but Lampard was angered that his side couldn’t build on Werner’s early goal.
‘We were dominant and we want to score more goals at that point because of the amount of possession. Couple of good chances, Callum had a good chance at the end of the first half,’ said Lampard.
‘Tottenham will have felt that hard and they changed their game to go longer in their build which meant that our fantastic press in the first half wasn’t as relevant up the pitch.
‘They turned us round and made us run backwards, rather than pressing forwards and that becomes really difficult to be fair to the lads. We could have been better on the ball in the second half, maybe dealt with the physical nature of the Spurs game but it slightly changed the case of the game.
‘They had chances in the second half but so did we. At 1-0, the second goal finishes it for us and we didn’t get that.’