GWANGJU, South Korea — Kristof Milak was 13 months old when Michael Phelps set his first world record, in the 200-meter butterfly. For most of the next 18 years in his native Hungary, Milak studied grainy videotapes of Phelps’s races, squinting at the images for tips on how to refine his butterfly technique.

The poor quality of the tapes precluded Milak from gleaning very much, he lamented. But that’s not how it looked to Phelps, who could not have been more impressed with Milak’s performance on Wednesday in the 200-meter butterfly final at the FINA World Championships.

In a race that Phelps watched online, Milak, 19, won in 1 minute 50.73 seconds, shattering the world record and becoming the first man not named Phelps to own it since 2001, a span of 18 years, 3 months and 24 days.

Milak finished the first 100 meters in 52.88 — the same split recorded by Phelps in 2009, when he lowered the record for the final time — then did the final 100 meters 0.78 seconds faster than Phelps.