Top-seeded John Nimick of Philadelphia held off a challenge from 17-year-old Kenton Jernigan yesterday to win the open championship of the United States National Squash Tournament in four games at the Capitol Hill Squash Club.

It was the fifth championship in as many months for the 23-year-old Nimick, who last week won the Canadian open championship.

"We play a lot together, and every time we do, he hits the ball harder," said Nimick, a 1980 graduate of Princeton. "I've played better than today, but he was hitting the ball so hard, it was all I could do to return it and wait for him to make errors."

Jernigan, a soft-spoken high school senior from Newport, R.I., who earlier upset the second and third seeds, powered his way back into the match after losing the first two games, 10-15 and 15-16. He hit several shots so hard that Nimick could only stand and watch them slam against the back wall, and won the third game, 15-4.

Nimick waited for Jernigan to make mistakes. After Nimick let a 4-0 lead get away in the last game, he came back to win, 15-11.

Boston's Thomas Poor, who won the 35-and-over age division in 1979 and 1980, regained the title from Len Bernheimer, 15-12, 15-3, 9-15, 15-12. Bernheimer, also of Boston, could have played in the 40-year-old division, but chose to defend his 35-and-over title instead.

Ian Dowdswell of Indianapolis won the 40-and-over division by defeating Robert Hetherington of Buffalo in three games, 15-7, 15-12, 15-6. Dowdswell trailed, 6-0, in the second game before he picked up some points on well-placed drop shots.

The only local winner in the nine divisions was John Weissenfluh of Potomac, who won the 70-and-over competition, held this year for the first time. Weissenfluh defeated 74-year-old Franklin Gould of Washington, the oldest man in the tournament, in five games, 15-9, 15-18, 17-14, 12-15, 15-9.