Having already lost to Canada, the US needed to beat El Salvador in Nashville in their Group A encounter on Monday and were moments away from completing the task.
The US had roared back from a 2-1 deficit to take a 3-2 lead, thanks to German-born forward Terrence Boyd's second strike of the match and a Joe Corona goal just four minutes apart midway through the second half.
The home fans' celebrations were silenced five minutes into injury time, however, when Jaime Alas scored for El Salvador to snatch a last four berth at the expense of their opponents.
Cuba, who drew with Canada earlier on Monday, were also eliminated.
Group B action concludes on Tuesday with Mexico having already claimed one of the berths and Honduras, Trinidad and Tobago and Panama battling for the last spot in the semifinals to be played in Kansas City.
The two finalists from the CONCACAF region will advance to London regardless of the result.
A late Oleg Zoteev winner shattered Syrian hopes and kept Uzbekistan in line for their first ever Olympic appearance with a 2-1 Asian playoff win yesterday.
Syria looked to have secured their second 1-1 draw in the three-team round robin in Hanoi, after Sunday's stalemate with Oman, until Zoteev's 88th-minute strike sent them tumbling out of contention.
Uzbekistan now need only a draw in the final fixture against Oman today to progress to an April 23 playoff against Senegal for a place at the London Games. The Uzbeks have three points and Oman have one.
Uzbekistan started the brighter but it was Syria who grabbed the lead when the unmarked Yasser Shahen ran on to a booming goal-kick, letting it bounce over his shoulder and calmly hitting a half-volley over Uzbek goalie Akbar Turaev on 14 minutes.
The Uzbeks regained their composure and they slowly turned the screws as striker Kenja Turaev glanced a header just wide before half-time. Zoteev also forced a diving save from Syrian stopper Ibrahim Alma after the break.
A long throw-in from the right finally unlocked the Syrian defense on 73 minutes as Temurkhuja Abdukholiqov flicked on at the near post and Turaev flung himself at the close-range header.
As the clock ticked down, Syria's Ahmad Aldouni fired wide with the goal begging and he was made to rue the howler when with two minutes to go, Zoteev coolly rounded Alma and steered the decisive shot home from a tight angle.
Meantime, New Zealand's Olympic qualifying hero Greg Draper yesterday described the goal that secured the team's ticket to the London Games as the "highlight of my life.”
The 23-year-old rising star of New Zealand football has scored 23 goals in the Welsh Premier League, where he plays for The New Saints, and represented New Zealand at the Beijing Olympics and the Under-20 World Cup.
But Draper, who has appeared for the All Whites national side as well as the Olympic under-23 team, dubbed the Oly-Whites, said his 17th minute penalty against Fiji trumped everything.
The goal secured a 1-0 victory in the Oceania zone Olympic qualifying final.
"It's the highlight of my life to be honest, scoring the goal to get New Zealand to the Olympics," he said as New Zealand become the 13th team to qualify for the 16-nation tournament.
New Zealand Football officials were yesterday seeking quick confirmation from the country's Olympic committee that the team will travel to London so they can begin planning their pre-Games preparation.
In Sunday's final in New Zealand, a hand-ball by Fiji's Ilisoni Tuinawaivuvu gave Draper the early chance to put New Zealand ahead and the Oly-Whites defended from there.
"We put our bodies on the line and defended like our lives depended on it, for the whole second half to be honest," Draper said.
New Zealand captain Adam McGeorge praised his side's defensive effort.
"It's finals football. It doesn't have to be pretty. Fiji threw everything at us. They definitely gave us a few wake-up calls but that just showed the resolves of the boys to get the result," he said.
