The Europeans broke the deadlock after just five minutes when Nani converted from close range, and they held that lead for an hour until Jermaine Jones blasted his side level with a magnificent strike.
Clint Dempsey looked to have earned his side a place in the last 16 when he completed the turnaround in the 81st minute, but Silvestre Varela converted Cristiano Ronaldo's cross with virtually the last action of the game to hand his side a lifeline.
Here, Sports Mole takes an in-depth look at another absorbing encounter at this remarkable World Cup.
On target: 5
On target: 7
Was the result fair?
The way it happened was certainly cruel on an American side that had put everything they had into the match, but in the end a draw is just about the fairest result tonight. USA had the better of things in general play, but they struggled to create as many good chances as they should have.
Most of Portugal's most promising moments came on the counter-attack as they failed to build on their early lead, but they were somewhat unfortunate to double their advantage just before half time as they hit the post and saw the rebound very well saved by Tim Howard.
USA did have other chances of their own, however, most notably when Michael Bradley saw a close-range effort brilliantly cleared off the line. If either side deserved to win then it was the Americans, but neither will have too many complaints with it ending all square.
Much like their display against Ghana, it was a mixed bag for USA today. They responded well to the early setback of going behind and looked more likely to score than Portugal for the majority of the first half. However, they also seemed to lack a cutting edge in the final third and were largely restricted to long-range shots.
They did very well to battle their way back into the match despite adopting a more defensive approach having drawn level with 25 minutes still to play. However, there was perhaps a touch of naivety late on as they failed to keep the ball, allowing Portugal to break and score the equaliser.
The manner of the result will be very hard to take considering they were a matter of seconds away from a place in the last 16, but there were still plenty of positives to take from the result. They now face a huge match against Germany and they will need to improve if they are to win that one, but they will certainly fancy their chances of advancing from the group.
Certainly an improvement on the shambolic defeat to Germany in their opening game, but still a long way from being good enough to reach the latter stages of this competition. They had a chance to really push on and make a statement having taken the lead so early, but they allowed USA back into the game.
They did defend well to prevent USA creating any really clear chances in the first half, but you feel that a top-quality side would put them to the sword, as Germany did. They struggled down their left flank all game and failed to rectify the problem, with both goals coming from that side.
They began piling the pressure on USA in the closing stages but, in truth, they didn't look like pulling an equaliser out of the bag until a superb cross from Ronaldo and a decent header from Varela saved the day. They still have an uphill struggle if they are to progress, but at least they have given themselves a glimmer of hope.
Sports Mole's man of the match
Clint Dempsey: Charged with leading the line in the absence of the injured Jozy Altidore, Dempsey was a constant thorn in the side of the Portugal defence. He was by far USA's most dangerous player in the first half, but a lack of support meant that many of his good runs proved to be fruitless.
He faded a bit in the second half as Fabian Johnson shone down USA's right, but Dempsey's refusal to give up was showcased as he bundled home what looked like being the winning goal. Another display full of passion, pride and quality from the midfielder.
This one came nice and early as Geoff Cameron sliced an attempted clearance into the path of Nani, who converted from close range for the opening goal. It was not a good delivery into the box, and Cameron should have comfortably dealt with it, but instead he put his side up against it from the fifth minute.
Nestor Pitana had a quiet game today, which can only be a good thing for a referee. There were no major decisions for him to make, and he only had to produce his yellow card once throughout.
USA: A draw will be enough for USA to progress now as they take on Germany in the group finale on Thursday.
Portugal: Portugal, meanwhile, would need either USA or Germany to lose and a substantial goal swing when they face Ghana to stand any chance of progressing.