Sorenstam, the pre-championship favourite, was upstaged by her compatriot, who compiled four birdies in a flawless four-under-par 67 in cool afternoon conditions following intermittent rain earlier in the day.
Neumann, the 1988 Open champion, was the only player in the 150-woman field not to make a bogey or worse as she matched her lowest round in a US Open.
She led by one stroke over the Americans Susie Redman, Kelly Robbins and Deb Richard, and the 19-year-old South Korean, Se Ri Pak.
Redman was helped by the first hole-in-one of her career, which came at the 15th, where her six-iron bounced three times and jumped into the cup.
Sorenstam found no such joy in her round as she shot a six-over 77 in a dreadful start to her quest to win the event for an unprecedented three consecutive years.
Laura Davies, who win in 1987, opened a gash in her left index finger while slicing some bread before the first round and she shot a 75 to stand eight strokes off the pace.Reuse content