Tseng, the 24-year-old Taiwanese star who has 15 LPGA titles and had a 109-week run at No. 1 in the world ranking, is winless in 36 events since the Kia Classic in March 2012. She has tumbled to No. 15 in the world and missed cuts in four of her last five tournaments.
After opening with rounds of 67 and 68, Tseng had 11 birdies and two bogeys Saturday in warm, dry and calm conditions to reach 18 under at Columbia Edgewater.
''It's been a long time. It's great to be back in this position again,'' Tseng said. ''I was really happy out there. ... I just love every part of golf out there and really enjoyed every shot.''
Tseng birdied six of the first eight holes and dropped a stroke on the par-4 ninth to make the turn in 5-under 31. She birdied No. 10, then hooked her second shot up against a restroom en route to a bogey on the par-5 12th.
Tseng rebounded with a birdie at 13, and added birdies at 15, 16 and 17 - all with putts of less than 10 feet. She squandered a chance for another birdie when she missed a 5-foot putt on the par-4 18th.
Tseng said the key to her round was making a 12-foot birdie putt at the par-3 13th after faltering at 12.
''That's an important putt for me, and I think kind of a little turning point on the back nine,'' Tseng said.
Tseng recently spent two weeks in Taiwan working on her game, and Tony Kao, her coach since age 6, is with her in Portland.
''I've been working really hard,'' Tseng said. ''I want to get back in this position, so I don't feel any surprise because this is what I'm working hard for.''
Norway's Suzann Pettersen, the 2011 tournament winner at Pumpkin Ridge, was tied for second with Thailand's Pornanong Phatlum. Pettersen had a 70, and Phatlum, the leader after each of the first two rounds, shot 71.
Pettersen has come from behind to win the Safeway Classic before, rallying from a nine-stroke deficit to win in 2011. Pettersen and Tseng are close friends, and will play together in the final group.
''There's no one that I'd rather see playing better and playing well than Yani,'' Pettersen said. ''I know she's been through a rough patch. She's too good not to hang tough and be in the game.''
Phatlum, the second-round leader for the first time during her five-year LPGA career, said her hot putter of the first two days deserted her.
''Maybe the pressure. Today is like slow, and I feel like not comfortable,'' Phatlum said. ''I'm just OK with my score today, and putting not really good. Just try my best tomorrow.''
Austin Ernst shot a 62 to tie the tournament record set by Annika Sorenstam in 2002.
Ernst set the stage as the first player on the course. Playing by herself, the tour rookie birdied eight of her first 10 holes to flirt with a 59, then bogeyed the par-4 11th. She was 12 under overall.
Ernst, who played her round in 2 hours, 45 minutes, said she went about low as she could go.
''I hit it really well, but I made a ton of putts. It was pretty easy on myself. Green were fresh,'' Ernst said. ''I maybe had two putts on the back nine that I could have made that were 15 feet that I just rolled over the edge. Other than that, I made pretty much got everything I could out of the round.''
Lewis said because of Ernst's early finish, most players were aware of her record-tying score.
''I was warming up on the putting green, and I think everybody on the putting green kind of stopped and watched her hit her birdie putt there on 18,'' Lewis said. ''Sixty-two is a pretty good score around this course.''
A 72-hole event for the time, the tournament is back at Columbia Edgewater after four years at Pumpkin Ridge. Columbia Edgewater also was the tournament site from 1990-2008.