Siriki Dembele actually shot Posh ahead from the penalty spot in the early stages after he’d been tripped inside the penalty area, but that just served to poke the beast.
England international Tammy Abraham equalised for Chelsea before United States international Christian Pulisic shot the hosts ahead from the penalty spot following a foul by Kyle Barker just before the break.
Moroccon international Hakim Ziyech was introduced as a half-time substitute and claimed a hat-trick, all with shots from inside the penalty area.
Posh played with plenty of spirit and there was a classy display from teenage defender Ronnie Edwards. Mo Eisa lobbed just over the crossbar in the first-half after racing clear, while Sammie Szmodics was denied by Chelsea ‘keeper Kepa Arrizabalaga.
Christy Pym denied Abrahams a second goal with a fine save, while Edwards produced a terrific block to keep out the same player.
Posh made 10 changes at the break with only Eisa starting the second 45 minutes while Chelsea also made changes with Ziyech among those introduced. Armando Broja claimed the final Chelsea goal.
Posh ‘keeper Dai Cornell made two smothering saves in the second period while the Blues had a goal chalked off for offside as they controlled the contest.
Chelsea (first-half): Kepa Arrizabalaga; Jake Clarke-Salter, Trevoh Chalobah, Malang Sarr; Davide Zappacosta, Danny Drinkwater, Ross Barkley, Marcos Alonso; Christian Pulisic, Callum Hudson-Odoi, Tammy Abraham.
Posh (first-half): Pym, Beevers, Edwards, Knight, Ward, Powell, Barker, Hamilton, Szmodics, Dembele, Eisa.
Posh (second-half): Cornell, Fernandez, Kent, Thompson, Butler, Broom, Burrows, Taylor, Grant, Eisa (sub O’Connell), Kanu.