Chelsea were on top for most of the first half and attempted a total of 16 shots, though both of their goals had more than a hint of fortune about them.
The Blues have suffered back-to-back league defeats and trail fourth-placed Tottenham by five points and seventh-placed Liverpool with nine matches remaining.
Chelsea will be looking to stay in touch with the Premier League's top four with a win over Crystal Palace at Stamford Bridge.
Willian stepped over a pass on the edge of the area, allowing Eden Hazard to take charge and feed Zappacosta into the right side of the box, with the Italian's attempt hitting Hennessey and going in off Kelly in calamitous fashion.
However, it's no use crying over spilled milk as we can only hope that Antonio manages to get his line up right and also decides to actually use his substitutes correctly this time, if need be.
Seven minutes later and the home side had doubled their lead with a goal equal parts lovely and gormless.
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Cesc Fabregas began it with an inch-perfect pass out to Marcos Alonso on the left.
At the other end, Chelsea failed to find a third - Willian had a shot blocked by the fearless James Tomkins and a free-kick turned away by Hennessey, who also denied Hazard.
"We tried to finish the game, kill the game, but I think that we tried and the goalkeeper made good saves".
"We hit the post".
Chelsea almost took advantage of that let-off but Tomkins threw himself in the way of yet another goal-bound shot, this time from Willian, and Hennessey saved from Hazard. "We are playing against one of the best teams in the world so we must pay great attention, having the same focus, the same attention as the first leg against Barcelona".
Speaking at a news conference ahead of Palace's trip to Chelsea on Saturday, he said: "Wilf's had a ball at his feet for a while, training solo with the rehab people and the physios".
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