Pfizer had previously cooled on Ratiopharm, but was asked to rejoin the auction and make a presentation to Ratiopharm's board and management, one source said.
Pfizer was expected to be competitive with a rival offer from Iceland's Actavis, the source said. Pfizer could still walk away without submitting a binding offer, the source cautioned.
The auction could linger several more weeks, with a decision unlikely before the end of the month, a second source said.
Several sources told Reuters in February that Teva Pharmaceutical Industries Ltd
That would make the deal the biggest generics takeover since Teva's $7.5 billion purchase of U.S. rival Barr, announced in July 2008.
Heir Ludwig Merckle put Ratiopharm on the block as part of concessions made to creditors by his father Adolf Merckle, who committed suicide in January 2009 after ceding control of his business empire to lenders during the financial crisis.
Pfizer declined to comment, while Ratiopharm was not immediately available for comment.
(Reporting by Jessica Hall, Ritsuko Ando in New York and Deena Beasley in Los Angeles; Editing by Steve Orlofsky and Phil Berlowitz)