Caeleb Dressel won the men’s 100-meter butterfly, setting a world record.

Credit...Doug Mills/The New York TimesDressel already held the world record (49.5 seconds, in 2019) and the Olympic record (49.71, Friday), and he came to the final as the favorite. No one else in the field went below 50 seconds in their preliminary heats.

Kristof Milak of Hungary, the gold medalist in the 200 fly, swam to Dressel’s left, and Dressel suspected that is where his closest competition would be. The two of them, and Michael Phelps and Milorad Cavic, were the only ones to swim sub-50 seconds in this event’s history.