Japanese flyweight boxer Kenji Kobayashi, 22, died yesterday, two days after being knocked down in his first professional fight.

An official said Kobayashi, hit by a series of punches to the face, was knocked down at the end of the fourth round of a scheduled four-round bout in Nagoya and lost consciousness.

Kobayashi is counted as the 21st professional boxer fatally injured in the ring in Japan -- the first since a national commission began conducting brain checkups last year in an effort to identify and retire high-risk boxers . . .

Hometown favorite Frankie Warren used an aggressive attack and a 12th-round knockdown to gain a unanimous decision over Houston's Ronnie Shields and win the vacant U.S. Boxing Association junior welterweight championship in a nationally televised bout at Corpus Christi, Tex. The World Boxing Council ranked Shields No. 2 and Warren only 13th in the 140-pound division.

Warren, 20-0 with 14 knockouts, dominated from the fourth round on, successfully getting inside to pound away to Shields' midsection. In the final round, a looping overhand right caught Shields on the left temple, dropping him.

Shields, 25-4 with 19 knockouts, is the North American Boxing Federation champion, but did not risk his title against Warren. He was taken to the hospital with what was initially thought a broken rib, but examination revealed only a severe bruise.

In a 10-rounder on the card, former Olympic lightweight Meldrick Taylor, 137 1/2 pounds, used a fast-punching attack to outpoint Harold Brazier, 139, in 10 rounds. Taylor is 12-0, Brazier 35-7.