Notes on this puzzle
Sudoku #5310 is a hard grid with 27 givens, which leaves 54 cells waiting for a digit. Played purely by logic, it takes 64 forced steps from the first placement to the last.
Most cells fall to the hidden single move: 54 of the 64 steps. Naked pair accounts for 4 more. The toughest moment needs box–line reduction, and that single step is what sets the level.
Among the givens, 1 is the most common digit (4 times) and 9 the rarest (1). The emptiest box is the top-left one, with 2 givens; the central box is the most crowded, with 5.
Where to beginLook at row 1, column 4 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.