Click here for full WEC FP2 results from the Circuit of the Americas
Porsche were made to sweat to maintain its dominance during the opening free practice sessions after the #8 Audi R18 e-tron quattro appeared to eclipse its rivals in the night practice, but Timo Bernhard snatched top spot with less than three minutes remaining.
Under the lights in Austin, Bernhard notched up the fastest lap of the weekend so far with a 1m 47.442s to go quicker than Audi by just over two-tenths of a second, while FP1 timesheet toppers #18 Porsche 919 Hybrid slotted into third ahead of the WEC championship-leading #7 Audi.
The sister Toyota LMP1 machines continued to trail its German rivals but was able to cut the deficit to 1.3 seconds as the temperature dropped at the Circuit of the Americas.
The biggest movers of the day were G-Drive Ligier's Roman Rusinov, Julien Canal and Sam Bird who jumped up the timesheet to head LMP2 and displace the dominant KCMG Oreca outfit.
The Porsche Manthay duo Patrick Pilet and Frederic Makowiecki topped the GTE Pro, while SMP Ferrari picked up the honours in the GTE Am class while outperforming a handful of GTE Pro machines.