One thin dime deprived Jan Stephenson of a tie for the lead with Nancy Lopez, Kathy Baker and Janet Anderson today after the first round of the U.S. Women's Open at Baltusrol Golf Club.

Lopez, who has struggled with nerves in this event, shot a two-under-par 70 on the hilly upper course to put herself in a tie with 24-year-old Baker and Anderson, the 1982 Open winner.

But Stephenson provided most of the day's drama when she was assessed a one-stroke penalty on the 11th hole after her ball marker, a dime, stuck to her putter. Angry and apparently close to tears, she took a bogey six, which cost her a share of the lead. She finished at a one-under par 71, tied with six others in second place.

"I'm usually so careful," she said. "It upset me and I couldn't put it out of my mind. It was a disappointing round because I was playing well, I felt confident and then all of a sudden I was shaken up. At first, I thought it was unfair, but we have rules."

Stephenson said she does not usually pat her ball marker into the green. The only reason she did on this occasion was because the new dime was giving off a glare she thought might have distracted Sheehan, who was preparing to putt at the 540-yard, par-5 11th. The coin stuck to the club for an instant, then dropped back to the green close to where she had first placed it. The rule states that a penalty is assessed whether or not movement of the marker is accidental.

Stephenson made par 4 on the 12th, but missed a two-foot putt on 13 for another bogey. She regained her composure, finishing with a string of pars and one-liners.

"I never had a penalty assessed for anything but hitting it out of bounds," she said. " . . . Tomorrow I'm going out there with a lead weight . . . I don't know what to learn from it, but that strange things happen. Except to Nancy Lopez."

Among those tied for second with rounds of 71 were Betsy King, 1984 player of the year; Lori Garbacz, who finished tied for third in last year's Open, and amateur Dotti Pepper, a 19-year-old Furman University sophomore who was low amateur last year. Also at 71 were Kathleen McCarthy, Judy Clark and Jackie Bertsch.

The 10 subpar first rounds broke the record of nine in the 1981 Open at LaGrange (Ill.) Country Club.

Seven players were two strokes back in third place at par including Amy Alcott, a two-time LPGA tour winner this year. They were followed by 10 at 73, including Beth Daniel and Patty Sheehan.

Hollis Stacy, the defending champion and three-time winner of the Open, shot a six-over-par 78 with a triple bogey at 13. Alice Miller, who already has broken the record for single-season earnings with $319,172 this year, bogeyed the second hole and double-bogeyed the third on the way to a 75.

Lopez, whose 32 tour victories do not include an Open title, had four birdies before finishing with a bogey at the par-4 18th hole when her second shot went over the green. After a slow start this season, she has won three of the last eight tour events, including the LPGA and last week's Mazda Hall of Fame Championship.

"I felt calm today, which I usually don't in Opens," Lopez said. "It's probably one of my best starts. Most of the time I play the Open like I'm in a foreign country. But I feel comfortable here . . . "

Three of her birdies came on the toughest, most sloping greens of the course, which is wrapped around heavily wooded Baltusrol Mountain. The first six holes, all built on the side of a hill with woods to the left, are generally acknowledged to be the most dangerous. But Lopez made a seven-foot putt on the par-4 second, chipped in the hole from 18 feet for another on the par-4 sixth, and got her third with a short putt on the par-4 ninth. She also birdied the par-3 No. 15 with a 10-foot putt.

Baker and Anderson had roller-coaster rounds with five birdies and three bogeys each.

Anderson, whose only tour victory came in 1982, played the first six holes one over when she bogeyed the third, fourth and sixth holes with three putts. But she birdied the first, fifth, eighth, and 11th holes, then finished the round with another birdie on the 18th by sinking a 20-foot putt.

Baker, the 1982 NCAA champion from Tulsa, struggled the most among the three leaders. She was four-under at the turn but found three bunkers in five holes on the back nine, bogeying the 11th and 14th.