Then skipper Gary Cahill scuffed a shot wide.
There was no such incompetence from Hull’s Mason, who wriggled through the Blues’ ranks to sting Courtois’ hands with a hefty low drive.
Chelsea got back on track after the break – they had gone three Premier League games without a win prior to this game.
Eden Hazard spelled danger when he saw his shot flipped away by Hull keeper David Marshall and then Costa finally got his act together.
He slipped past Marshall and his shot for goal hit Jake Livermore and the post, before the ball fell for N’Golo Kante who ballooned over.
But in the 61st minute Chelsea broke the deadlock. Willian, just inside the box, showed his first glimpse of real class with a curler into the top corner to leave Marshall thumping his thighs in anger.
And six minutes later, Costa picked his spot in similar fashion with unhappy Marshall powerless to stop a second sucker punch.