Why these puzzles are so demanding
Extremely hard Sudoku puzzles often require several connected deductions. Instead of guessing digits, compare candidates and look for patterns spanning different rows, columns and boxes.
The number of given digits alone tells you little about difficulty. The real challenge depends on which logical techniques are needed to advance.
When simpler deductions stop working
Truly extreme puzzles can require lengthy chains of deductions. Use accurate pencil marks and take your time. If you get stuck, a hint can help you identify the next step.