The Forsa institute said both parties were now tied on 222 with Angela Merkel's conservatives.
The Infratest dimap institute released the same estimate of a draw between the two biggest parties following the inconclusive election, if the SPD manages to win extra seats granted based on Germany's complex proportional voting system.
Although the Social Democrats scored fewer votes than the Christian Democrats, it can win extra seats.
Schroeder and Merkel both immediately insisted they each had won a mandate to form the next government.
Merkel's preferred coalition partners, the pro-business Free Democrats, tallied a surprisingly strong result of around 10 percent, but it was not enough to form the center-right majority Merkel had said she needed to jumpstart the stalled economy and help the 4.7 million jobless back to work.
Tipping the scales were so-called "overhang" seats, which derive from the two votes that each German casts when he goes to the polls.
In the first vote, the candidate with the most votes in each constituency is elected to one of 299 seats - half of the 598-seat Bundestag chamber.
The second vote decides the allocation of the other 299 seats via the state lists using a proportional system of calculation.
Those seats are distributed based on the number of votes obtained by party lists, with people at the top of their lists having the best chance of getting in.
Overall, it means a party can win seats individually and as a group, and if there is a difference in the number obtained under either system, the party is allowed to keep the extras, which are known as the "overhang" seats.
As a result, the Bundestag elected at the last general election in 2002 had a total of 603 instead of 598 seats.
Once the Bundestag has been chosen, the federal chancellor is elected with an absolute majority of parliamentary votes.
Merkel said she had the upper hand, despite failing to secure a ruling majority, and that she would negotiate with all the major parties on forming a coalition.
"We have a very clear mandate and I assume this mandate with all my power," Merkel told cheering supporters in the capital. "We now need to form a stable government."
But the charismatic Schroeder, 61, insisted he had been confirmed in power.
"I feel I have won approval to provide a stable government for the next four years under my leadership," he told a euphoric crowd at the SPD building, his hands clasped above his head like a boxing champion.
The results mean the parties led by Merkel, who had been tipped to win the election and become Germany's first woman chancellor, and Schroeder's SPD may be forced into an unwieldy grand left-right coalition - a prospect she has labelled a recipe for gridlock.
Schroeder said his party, energised by a result that beat most expectations, would never enter into a coalition under Merkel.
"Mrs Merkel will not obtain a coalition with the SPD if she wants to become chancellor," he said in a round-table debate with the main candidates.
He was counting on winning over the Free Democrats (FDP) to join his Social Democrats and the Greens, junior partners in the current government, forming a so-called traffic light coalition for the party colors red, yellow and green.
But FDP leader Guido Westerwelle, recognising his role as likely kingmaker, vowed not to abandon Merkel in favour of Schroeder and was counting on a shift in the results as the last ballots were counted.
"We hope that as the evening wears on, we will have enough to govern with the Christian Democratic Union ... if not we will remain in the opposition," Westerwelle told supporters shortly after voting ended in a general election.
The FDP polled at just over 10 percent, gaining some three percent on their last performance in 2002 elections to make them the third strongest party.
The Greens reached about eight percent, better than forecast but not enough to save the government in its current form.
A new alliance of Social Democrat dissidents and former communists known as the Left Party garnered around 8.5 percent, effectively robbing the mainstream parties of key support in an extremely close election.
A three-way left-wing coalition featuring the Social Democrats, the Greens and the Left was ruled out by each of the parties, mainly due to the bad blood with the Left Party's chief candidate Oskar Lafontaine, a former SPD leader and finance minister who resigned.
Merkel, too, refused to negotiate with the far left, saying she planned to "talk to all political parties, except the Left Party" in her attempts to form a governing coalition.
Nearly 62 million Germans were called to the polls after Schroeder forced through the election 12 months ahead of schedule in a bid to obtain a fresh mandate for his disputed economic reform drive.