Vijay Singh reeled off five birdies in six holes in a sizzling round of 63 to join Jim Furyk at the top of the US Open clubhouse leaderboard.
Defending champion Tiger Woods lies three shots back after a second-round 66.
Singh, who eagled the first on his way to a front nine of 34, picked up shots on 10, 11, 13, 14 and 15 to get to seven under for the tournament.
But the Fijian missed a golden chance to become the first man to shoot a round of 62 in a major championship.
Singh, who is one of 20 players to have previously recorded a 63, missed 12-foot chances on the 16th and 17th and then drove into sand on the 460-yard 18th.
The former Masters and PGA champion had to settle for another par - and joined Greg Norman as the only players to have two rounds of 63 in majors.
American Furyk had earlier set a new 36-hole record for the US Open with his rounds of 67 and 66.
LEADERBOARD (US unless stated)
-7 V Singh (Fij), J Furyk
-5 S Leaney (Aus), J Byrd
-4 N Price (Zim), E Romero (Arg), F Jacobson (Swe), T Woods, J Leonard
-3 T Watson, R Damron
Two shots back in third place is Australia's Stephen Leaney, whose bogey-free round included birdies on 18 and at the first, and Jonathan Byrd.
In benign conditions at an overcast Olympia Fields Country Club, Furyk birdied the par-five first after hitting his approach to within two feet, and then picked up further shots on six, 13 and 14.
World number one Woods, who eked out a scrappy 70 on Thursday, got off to a flier by sinking long-range birdie putts at the first and fourth holes to get to two under.
Although the 27-year-old, bidding for his third US Open crown in four years, bogeyed the fifth after his approach spun sharply back off the front green, he recovered with an easy birdie-four at the sixth.
Another birdie followed at the par-four ninth, where he rifled his approach to just three feet, but he missed the green with his second shot at the 467-yard 11th and failed to get up and down to salvage par.
Further birdies on 13 and 16 lifted Woods to four under.
Veteran Tom Watson, who fired a scintillating five-under-par 65 on Thursday, had been two strokes off the second-round lead with six holes to play.
I was just playing a hole at a time and I got a momentum going, but I would feel even better if I did it on Sunday
But the 53-year-old American ran up a three-putt double-bogey at the par-four 12th to slip back.
Scotland's Colin Montgomerie had a disappointing second round, although he did succeed in making the cut - something he had failed to do in his last three tournaments.
Montgomerie, who refused to speak to waiting reporters after the second round, is three over after a 74 which featured three successive bogeys after the turn and another at the 15th.
Padraig Harrington was also far from his best in a second round of 72 and he is one over along with Justin Rose who shot a 71.
Twenty-four players broke par in the first round, the third highest figure in US Open history, and the halfway cut on Friday was a three-over 143.
Among the notable players to miss out were former majors champions Jose Maria Olazabal (144), Nick Faldo and Rich Beem (150), Davis Love III (151) and 2001 British Open winner David Duval (seven over after six holes).