As Storm Bert batters British shores, the West London giants triumphed in challenging conditions in Leicestershire, as Fernandez teed up Nicolas Jackson before putting the tie to bed in the second period, where Jordan Ayew's injury-time penalty came too late for the hosts to launch a magnificent comeback.
Reuniting with the club he led to the Championship crown in the 2023-24 campaign, Maresca witnessed his current employers dominate proceedings from the off, and the deserved opener arrived from the foot of Jackson in the 15th minute.
The Blues' slick passing and intricate link-up play was penning Leicester in, and Steve Cooper's men responded by scything the visitors down; all of Wilfred Ndidi, Boubakary Soumare and Oliver Skipp, who replaced an injured Harry Winks, were booked in the first half.
A different referee may have deemed Ndidi and Soumare's terrible challenges to be red-card worthy, but Andy Madley kept it 11v11, allowing Leicester to pile the pressure on Chelsea and an unconvincing Robert Sanchez as the first half drew to a close.
Normal service resumed at the start of the second period, where Cole Palmer could not help but laugh after his one-yard tap-in was blocked by his own man in Noni Madueke, but the pair would not be left to lament that comical mix-up.
Indeed, Fernandez went from provider to goalscorer with 15 minutes of normal time remaining, nodding home after Jackson's header was saved, and the Blues survived a huge scare in the final few moments after Ayew's stoppage-time spot kick.
While Chelsea stay third in the Premier League table after Saturday's success, they have cut the gap to Manchester City down to just one point, while Leicester occupy 16th with just a three-point buffer over the relegation zone.
LEICESTER VS. CHELSEA HIGHLIGHTS
Nicolas Jackson goal vs. Leicester (15th min, Leicester 0-1 Chelsea)
It had been coming.
Wout Faes and Jackson are engaged in a tight tussle for the ball going back towards the Leicester goal, and the Chelsea striker shows terrific strength and desire to win his battle with the Belgian defender, who in truth should have done a lot better.
Jackson plays a one-two of sorts with Enzo Fernandez, skips past Caleb Okoli and conjures up a delightful outside-of-the-foot finish into the far corner.
Enzo Fernandez goal vs. Leicester (75th min, Leicester 0-2 Chelsea)
A belated second for the Blues, and the three points are surely in the bag now. Maresca's men build up patiently around the Leicester box before Marc Cucurella sends a cross in to Jackson, who outmuscles Okoli and sees his header saved by Mads Hermansen.
However, the Leicester goalkeeper can only palm Jackson's shot into the path of Fernandez, who cannot miss with a simple header of his own from a few yards out.
Jordan Ayew penalty goal vs. Chelsea (90+5 min, Leicester 1-2 Chelsea)
A very, very late flicker of hope for the Foxes, as Bobby Decordova-Reid is tripped by Romeo Lavia inside the area; the linesman's flag initially gives Chelsea a reprieve, but a VAR check shows that he was indeed onside and a penalty is awarded.
With Jamie Vardy off the pitch, Ayew takes a short run-up and manages to find the bottom corner with the help of Sanchez's palm.
MAN OF THE MATCH - ENZO FERNANDEZ
Winning his place in the starting lineup back from Lavia, Fernandez more than repaid Maresca's faith in him with one of his best performances in a Chelsea shirt in recent memory, setting up Jackson's opener before finding himself in the right place at the right time to rubber-stamp Chelsea's win.
The Argentine's assist was one of three chances he created on the afternoon, and he was just as fundamental defensively, putting in five tackles and two clearances in a midfield masterclass.
LEICESTER VS. CHELSEA MATCH STATS
Possession: Leicester 36%-64% Chelsea
Shots: Leicester 4-16 Chelsea
Shots on target: Leicester 1-7 Chelsea
Corners: Leicester 2-9 Chelsea
Fouls: Leicester 15-12 Chelsea
BEST STATS
WHAT NEXT?
Chelsea now turn their attention back to the Conference League with a trip to Heidenheim on Thursday, three days before they host Aston Villa in the Premier League.
As for Leicester, Cooper's men have a week to conduct their post-mortem before welcoming Brentford to the King Power next Saturday at 3pm.