Sweden's Alexander Noren, buoyed by birthday flowers from his golfer girlfriend Emelie who won a tournament in Norway this week, fired a second consecutive 66 to share the halfway lead with overnight pacesetter Francesco Molinari (70).
The pair were 12-under 132, one shot ahead of Argentine Ricardo Gonzalez (64) and young Italian Matteo Manassero (64), who would qualify for next week's British Open with a top-five finish.
American four-times major winner Mickelson (137) started in ideal fashion by holing out from 60 yards for an eagle two at his first hole.
Mickelson's previous eight rounds had all been over par, a career first and not the form the 42-year-old expected heading into the third major of the year at Royal Lytham & St Annes in England from July 19-22.
"I'm really glad I added the tournament and it's very helpful to see my game progress so quickly," Mickelson, who only entered the event on Saturday, told the European Tour website.
"In fact, when I was thinking about adding this tournament, after missing the cut last week, my wife Amy and I thought simultaneously of the same decision, so I am very lucky."
Noren started birdie-birdie-double bogey but an eagle at the par-five 6th hole was the highlight of his four-under back nine.
Noren thanked his girlfriend for the unexpected gesture.
"That's the first time I've had flowers," said the world number 89, who turned 30 on Thursday. "Emelie gave me a nice birthday gift in winning a tournament on Monday, so that was a really nice present for the both of us."
Britain's world number one Luke Donald and 14th-ranked Martin Kaymer of Germany carded four-under 68s to lie three off the pace on nine-under as they seek to fine-tune their games before next week's British Open.
Manassero, who finished 13th on his Open debut three years ago at Turnberry but missed the cut last year in Sandwich, said playing links golf in a major again was the only thing on his mind.
"This is my last chance (to qualify). I would really love to be in The Open. I like links golf, it has a special atmosphere," said the 19-year-old twice European Tour winner.