The pair fired 8-under par 64s to set the course record at Squaw Creek Country Club. The scores also tied the tournament's 18-hole mark and left Robbins and Bader one shot ahead of Mi Hyun Kim and Danielle Ammaccapane.
Rookies Natalie Gulbis and Jung Yeon Lee finished in fifth place with 5-under 67s.
Robbins, a nine-time winner in 10 years on the LPGA Tour, tallied six birdies and two bogeys Friday. She also made a pair of eagles -- the first when she holed out a 50-foot bunker shot at the 479-yard, par-5 5th hole, and the second with a four-foot putt set up by a driver and 3-wood to the green at the 486-yard eighth.
She used her wedges as weapons on Friday, with four of her half dozen birdies coming on putts inside six feet. In fact, three were from three feet and in.
'It's good to get off to a really nice start,' said Robbins, 32, who hasn't posted a victory since the season-opening event of 1999. 'I've been playing really well, I just haven't been making a whole lot of birdies.
'I just haven't done a lot of really good things to put me into any type of real contention.'
Robbins has logged two top-10 finishes in 2002 -- a tie for third at the Welch's/Circle K Championship and solo second at the Chick-fil-A Charity Championship.
Her first-round 64 tied her career-low round. She reached the same score seven times previously.
Baders 64 was the best of her short career. The 28-year-old, who joined the LPGA Tour last year, shot a 66 for a share of the lead after one round of the Corning Classic back in May. She followed with 77 in the second round and in the end tied for a disappointing 65th.
'It's my second year,' Bader said. 'I need to chill out, relax and have fun with it. The more upset and frustrated you get, the harder it is to play. No point in pouting all day -- I decided to change my attitude a little bit.'
Bader birdied her first three holes, starting on the 10th, but three-putted for bogey at the par-5 13th. She rebounded with three more birdies at 14, 17 and 18, then went to 6-under by wedging her third shot to two feet for birdie at the par-5 second.
Though she failed to get up and down out of a bunker and bogeyed the fifth, another par-5, Bader cleaned up with birdies on three of her last four holes.
Eight of Bader's 10 birdie putts were holed from 10 feet and in.
Both Robbins and Bader were on the good side of the draw on Friday. Their morning tee times kept them ahead of the lightning and heavy rains that delayed play for more than two hours in the afternoon. As it turned out, only two of the top 14 scores were posted after the storm came through.
Jackie Gallagher-Smith, who earned her lone victory at the 1999 Giant Eagle, shot 68 and was part of an eight-way tie for seventh place at 4-under par.
Two-time defending champion Dorothy Delasin, looking to become only the seventh player to win the same LPGA tournament three years in a row, turned in a 3-under 69 and was tied for 15th.
Full field scores from the Giant Eagle LPGA Classic