World championship bronze medallist Dmitriy Karpov of Kazakhstan was leading the Olympic decathlon at the halfway point on Monday after a consistent all-round display in the opening five disciplines.

Karpov leads the overnight standings on 4,689 points with world record holder and Sydney silver medallist Roman Sebrle of the Czech Republic second on 4,594.

American Bryan Clay, who led after the first three events, slipped back to third on 4,554, with Briton Dean Macey fourth 100 points back and world champion Tom Pappas of the United States fifth on 4,415.

Former world record holder and three-times world champion Tomas Dvorak was forced to pull out through injury after the first event, the 100 metres, when he aggravated an Achilles tendon injury.

Clay got the day off to a high-quality start in the sprint, picking up 989 points when, aided by a 2.2 metre-per-second following wind, he equaled the second-fastest decathlon 100 in Olympic history with a run of 10.44. Only American Bill Toomey's 10.41 en route to gold in 1968 has bettered it.

Clay's 7.96-metre long jump was also the best of the day but he cut it fine in the shot, landing 15.23 metres with his third throw.

Karpov was always in touch, however. His 10.50 sprint was the second best, his 7.81 long jump and 15.93 shot both the third.

A 2.09 high jump lost him some ground but he stormed back by smashing his personal best with a stunning 46.81-second 400 metres, the best of the field by more than a second and worth 968 points.

Sebrle, the only man to break the 9,000 points barrier, had a steady start with a 10.85 sprint and 7.84 long jump but closed in with a personal best 16.36 shot.

He then cleared 2.12 metres in the high jump and finished well with a 48.86 400 metres.

Clay dropped back with mediocre showings in the high jump and 400, allowing Macey to close in.

The injury-ravaged Briton, fourth in Sydney, matched his personal best with the best high jump of the day, 2.15 metres.

After collapsing to the floor following his 48.97 400 he received a standing ovation.

"Not once have I had that before, it brought me to tears -- nearly," he said.