Notes on this puzzle
27 digits are printed in this grid and the other 54 cells are yours to fill. A solver that never guesses gets through it in 55 steps.
Most cells fall to the hidden single move: 52 of the 55 steps. Single candidate accounts for 2 more. The toughest moment needs pointing pair, and that single step is what sets the level.
Among the givens, 3 is the most common digit (4 times) and 9 the rarest (1). The emptiest box is the top-left one, with 1 given; the top-right box is the most crowded, with 4.
Where to beginLook at row 5, column 2 first: a hidden single is available there from the very start. Pencil in candidates for one box at a time and watch for two cells that share the same pair.