Plenty of players still sit in contention at Merion, but that daunting closing stretch of holes means that nothing will be decided until the very end on Sunday.
The likes of Luke Donald, Justin Rose, Charl Schwartzel and Hunter Mahan are all in striking distance, but the top two in the world struggled again.
Here we get the views of the big names after Saturday's efforts.
The five-time runner-up has looked like the winner from the first round, and he is still looking relaxed as he insists the final day will be fun.
There will be no 15th major for the world No 1 as he shot a six-over 76 - his worst round as a professional in the US Open - to slip way down the pack. Woods was left frustrated again after fancying his chances of challenging.
The Englishman got himself into contention again but again he struggled over the last few holes to drop back and now sits six shots behind Mickelson - saying he might need a padded cell.
McIlroy started his third round with a birdie but then sliced his drive out of bounds on the second and that set the tone for a third round of 75 to slip nine shots behind. McIlroy said the flaws in his game had been exposed by Merion, but he has looked to be swinging a lot better and the signs for the rest of the year could be promising.
Luke Donald has led at one stage in all three rounds, but after slipping two shots back late on he insists he is focusing on the positives heading into Sunday. Those closing holes will be tough to get right if he is still in with a shout late on the final day though.
Sergio will not want to see Merion's 15th hole again after Sunday. He scored an eight there on Thursday and on Saturday he hooked three drives out of bounds before eventually finishing the hole and carding a 10. Garcia did manage to shoot 75 even after that to his credit but after an eight and 10 who knows what the 15th will bring on Sunday?
Rose felt 'hard done by' after he dropped shots at the final two holes which would have seen him share the lead with Mickelson. The Englishman is still well in contention though and said he felt calm all the way around Merion on Saturday.