Hall, with an 8-under 136 total, leads Laura Davies, who shot a 70. Overnight leader Louise Stahle had a 74, slipping into a tie for third place.
Annika Sorenstam tallied her second straight 70, but Natalie Gulbis (75) made an early exit, missing the cut of 6-over 150.
Earlier this season, the 40-year-old Hall won the Northern Ireland Ladies Open after an eight-hole playoff for her first win in 11 years. She won again at the Nykredit Masters.
'The turnaround in form is down to a lot of things,' Hall said. 'A good caddy, good fortune, good work, a lot of effort, perseverance, enjoyment, perspective. ... It's been a long journey.'
Davies closed with three straight birdies.
'It was a bit slow on the golf course, which threw us off a bit,' Davies said. 'But it was nice to finish the way I finished after a shaky start.'
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