The dismal result comes amid grumbling within Westerwelle's own party about his performance and suggestions that a poor result in an upcoming state election could even be a death knell for his leadership.
The poll published by Stern magazine and broadcaster RTL revealed Westerwelle's pro-business FDP had shed a further point even after dropping steadily for weeks.
That means the party has lost half the 14.6 percent support it enjoyed during its triumphant performance in the federal election last September.
Westerwelle, dogged by weeks of criticism that his party had been ineffective in pushing its tax-cutting agenda as junior coalition party to Chancellor Angela Merkel's conservative Christian Democrats, has gone on the offensive in recent days, attacking Germany's welfare system and accusing his critics of acting like socialists.
Westerwelle said he was unconcerned by fluctuating poll results, which he said were a normal part of the political cycle.
“I'm now in my ninth year as party leader. We've gained in every federal election since then,” Westerwelle told the Passauer Neue Presse. “Election results matter. That counts.”
But one senior party member has told news magazine Der Spiegel that if the party bombed in the May 9 state election in North Rhine-Westphalia, Westerwelle's position would be under threat.
“The party has always enjoyed success with Westerwelle. But he's not an enduringly unifying figure,” said one leading member of the party. “It it goes badly for the FDP in North Rhine-Westphalia, we'll have a debate about personnel.''
That would mean a discussion about separating the roles of vice chancellor and foreign minister from the position of party leader. Traditionally, those two jobs in the government go to the leader of the junior coalition party.
Merkel's CDU picked up a point to move up to 35 percent and the opposition Social Democrats remained unchanged on 22 percent for the third week running.
The Greens are still on their record high of 17 percent while the socialist Left party gained a point to sit on 12 percent. Miscellaneous parties lost a point between them to hold 7 percent.
That means if an election were held tomorrow, a coalition of the SPD, Greens and Left could easily defeat a centre-right alliance between the CDU and the FDP.
The poll was conducted by Forsa surveying firm last week, which means the results were at least partly affected by the quarrelsome debate over welfare, which kicked off last Tuesday after the Constitutional Court ruled that the Hartz IV system of unemployment benefits were unconstitutional.