Clay, the 2005 world champion, was second overall in the 110-meter hurdles in 13.93 seconds for 984 points, then won the discus with a throw of 53.79m for 950 more and cleared 5.00m in the pole vault for 910 points.
That left Clay on 7,365 total points with only the javelin and 1,500m to come in the two-day, 10-discipline competition that began Thursday with the 100m, long jump, shot put, high jump and 400m.
"I started the day pretty tired, as everybody does," Clay said. "My first race went okay. I didn't fall and I had an okay time so I will take it."
Andrey Krauchanka of Belarus, this year's world indoor runner-up, stood second on 7,049 with Alexander Pogorelov of Russia third on 6,979.
Ukraine's Oleksiy Kasyanov, second by 283 points to Clay when the pole vault began, cleared only 4.30m and fell to fourth on 6,874 as Clay shared third in the vault with Krauchanka and Pogorelov.
A nightmare run for US athletes, including Jamaican sprint domination and two botched final handoffs in the 4x100m relays, continued when Trey Hardee, fourth after seven events, scored no points in the pole vault.
Hardee, with the third-best decathlon total score in the world this year, missed three times at his opening height of 4.70m to doom his medal bid.
Reigning world and Olympic champion Roman Sebrle of the Czech Republic saw his podium hopes fade as well.
The injury-nagged 33-year-old world record holder was third in his 110 hurdles heat in 14.71, reached only 45.50m in the discus and 4.80 in the pole vault to stand on 6,823.
Clay completed the opening day with an 88-point edge over Krauchanka and began the day by winning his morning 110 hurdles heat in a head-to-head showdown with Hardee, fifth in 14.20, and Krauchanka, sixth on 14.21.
Clay boosted his lead to 159 points in the discus, where he owns the world decathlon best of 55.87m, by making his mark on his first attempt.
Kasyanov waited until his last throw to reach 48.39m for 837 points and move into second past Krauchanka, who threw 44.58, and Hardee, who threw 43.55.
Clay, 28, achieved the world's highest decathlon score since 2004 at the US Olympic trials, his personal best of 8,832 points the top US result in 16 years.
