Defending champion Clay appeared to misstep when he planted his foot for his second attempt in the high jump at 2.00 meters, and slumped under the bar onto the mat. He limped away, and did not come back for his third attempt.
The American's status for the 400 meters, the fifth and last event of Friday's decathlon events, was unclear.
Jamaican Maurice Smith also failed at the same height and could not further build his lead in the standings. Czech Olympic champion Roman Sebrle however moved up into second place when he cleared a season's best 2.12 meters.
After 4 of 10 events, Smith had 3,591 points for a 28-point edge over Sebrle. Clay faded to third with 3,558.
Clay's misfortune in the high jump spoiled a good opening to the competition. He started with victories in the 100 and the long jump. But he slipped behind Smith after the shot put.
Having painted the distinctive red circle of the rising sun on a white field on both his shoulders, Clay wanted to show his kinship with the crowd.
"I am half Japanese and I wanted to let them know that we share the same blood," he said.
After failing to defend his 1,500 title and finishing with silver, Ramzi could not even make the final of the 800 on Friday. In his semifinal, he faltered badly and finished last of his race. Favorite Yuri Borzakovsky easily won his semifinal to advance to Sunday's final.
While Clay and Ramzi had trouble in the stadium, for Russia, it was a walk in the park earlier Friday.
Olga Kaniskina led a 1-2 finish in the women's 20-kilometer through the muggy parkland outside the Nagai stadium, keeping Russia in close contention with the United States in the medal standings.
And Russia had to do it without defending champion Olimpiada Ivanova, who walked out early when the pain of an old skiing accident became too much.
Instead of 37-year-old Ivanova, a new generation took over. Kaniskina, 22, who walked away from the pack early, was never troubled again. Behind her, 19-year-old Tatyana Shemyakina won silver.
"To be world champion after 2 1/2 years of serious walking training is great," said Kaniskina.
The result improved Russia to 12 medals, including four gold and just one medal behind the United States, which has 13 overall, including 6 gold.
In the high jump, season's leader Blanka Vlasic of Croatia was among 16 who reached the qualifying mark of 1.94. Defending champion Kajsa Bergqvist of Sweden, Olympic champion Yelena Slesarenko and European titlist Tia Hellebaut also advanced.
In the javelin, defending champion Andrus Varnik of Estonia failed to qualify for the final.
Vadims Vasilevskis of Latvia had the top throw of 87.37 meters. Season's leader Breaux Greer of the United States, Olympic champion Andreas Thorkildsen and Finland's Tero Pitkamaki also advanced.
Later Friday, Liu Xiang was favorite to give China its first gold of the championships in the 110-meter hurdles.
Veronica Campbell was looking for a sprint double to match Tyson Gay's 100-200 gold medals in the men's sprints. Defending champion Allyson Felix looked strongest though in qualifying.
Tatyana Lebedeva was looking for a double of her own. After the Russian won the long jump, she is favored to add the triple jump too.
And if anyone is expected to have an easy run toward gold, it is Jeremy Wariner in the 400, an event the defending champion has dominated for years.
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