SAINT-DENIS, France -- Dmitry Karpov of Kazakhstan set three personal bests and matched another Tuesday to take a 50-point lead over American Tom Pappas halfway through the decathlon at the World Track and Field Championships.

World record holder Roman Sebrle of the Czech Republic, strongest in the second day of competition, was in third, 176 points behind the leader.

Bryan Clay of the United States was third through four events but pulled up in the final event, the 400 meters, with a right hamstring injury and withdrew from the competition.

Pappas, trying to become the first American to win the decathlon at the worlds since Dan O'Brien in 1995, held off Sebrle to win his 400-meter heat in a personal best 47.58 seconds. Pappas' run came after a disappointing high jump of 6 feet, 10\ inches

"I was very, very pleased with that performance,'' Pappas said of the 400. "I was a little ticked off from the high jump. I wanted to end the day on a good note and build a little momentum going into tomorrow.''

Through five events, Karpov had 4,599 points, including personal bests in the 100, shot put and 400. He matched his best in the high jump. Pappas was at 4,546, followed by Sebrle's 4,423. Claston Bernard of Jamaica was fourth with 4,239.

Pappas should have an advantage over Karpov on the second day, mainly because of a wide difference between the two in the javelin. Pappas has a best of 216-11, Karpov 165-9.

Pappas, however, said that how he does in the other events will determine whether he can win the gold.

"The discus and the pole vault will be pretty big, "also the 1,500,'' he said.

That 1,500 is always the most grueling, coming at the end of two exhausting days.

"Being out here for this long, you've got to be really mentally tough,'' Pappas said. "It's so easy to tell yourself, `Man, I'm just ready for this thing to be over with.' "

Pappas has been chasing Sebrle for two years. In Gotzis, Austria, this year, Pappas led the Czech after eight events, but Sebrle charged back for the victory. Pappas finished with a personal best 8,585 points to Sebrle's 8,807. Pappas increased his best to 8,784 at the U.S. championships at Stanford on June 22, then targeted Sebrle at the worlds.

"I haven't put a good second day together in a long time,'' Pappas said. "If I can do that, I think I can surprise some people.''

U.S. teammate Paul Terek, in fifth place with 4,222 points, has his money on Pappas.

"I think Tom's going to win this,'' Terek said. "This is Tom's meet. Tom's an athlete. He's a warrior. It showed in the 400.''