Schafer, who has never won a medal at senior level, tops the standings on 4,036 points with last year’s Olympic champion and favourite Nafi Thiam not far behind on 4,014.
Schafer’s 13.09 seconds in the 100 metres hurdles gave her an early advantage over her Belgian rival, but Thiam finished first in the high jump and the shot put to accrue valuable points.
"I'm very happy with my first day especially as I came into this in bad shape. I'm looking forward to the next three events,” the 25-year-old Schafer said.
Thiam remained confident.
"The challenge is hard but you have to be able to be solid but have a big performance on a few events," she said. "It is easy to make a mistake in seven events."
There was disappointment for home favourite Katarina Johnson-Thompson, who is bidding to emulate her compatriot and three-times world champion Jessica Ennis-Hill.
She came sixth in her favourite event, the high jump, but brought herself back into contention in the 200m, winning her heat in 22.86 seconds to finish the day fourth on 3,838 points, behind Cuba’s Yorgelis Rodriguez on 3,905.
The heptathlon concludes on Sunday with the long jump, javelin and 800 metres.
(Editing by Ed Osmond)
