Ultra-low-voltage (ULV) logic offers the opportunity to operate at the minimum-energy point (MEP) for applications with low-to-medium speed requirements. Unfortunately, the critical design constraint of achieving a reliable timing closure at the target frequency of the application becomes very complex in the wide design space of ULV including supply (Vdd) and threshold (Vt) voltage selection as well as netlist optimizations from the synthesis. In this paper, we propose a fast synthesis flow to accurately predict the Vdd/Vt MEP under strict timing constraints. Compared to an exhaustive search for the MEP under timing constraints based on numerous library recharacterizations and synthesis steps for all Vdd/Vt pairs, the proposed ULV flow dramatically speeds up the design process. Indeed, it requires a single library recharacterization and only three synthesis steps. Results obtained for several ITC''99 benchmarks under a wide range of timing constraints from 0.1 to 30 MHz in 65-nm LP/GP CMOS demonstrate that the proposed flow has a less than 10\% energy penalty with respect to the absolute MEP computed with an exhaustive search and energy savings enhanced up to 2.4\texttimes compared to a conventional flow with Vdd scaling only.