The Blues started with N'Golo Kante and Eden Hazard on the bench and were outplayed for long periods until Alonso headed in an unlikely winner four minutes from time.

Victory was scarcely deserved for Chelsea, with Kepa Arrizabalaga starring in the away goal and the notion of a kind draw for the visitor put to bed by an excellent Slavia display.

But Maurizio Sarri's decision to rest key men was ultimately justified as his side prepare to visit Liverpool on Monday (AEST) in complete control of this European tie ahead of next week's return leg.