Willis ran a tactically astute race to cruise home in third 3m 34.8s in easily the fastest semifinal.
Willis, 29, even had time to glance at the trackside clock coming down the home straight to ensure he was in.
The Michigan-based runner looks in superb shape. He jogged back around the outer afterwards, just to let his rivals know he has a lot more left in the tank.
A medal is no certainty, it might not even be a probability given the class of the field, but Willis is doing everything right as he looks to add to the silver he won four years ago.
He was more than happy to sit back early and let Silas Kiplagat set a torrid pace. He moved out with ease midway through the third lap and made sure he was never near any flailing elbows or spikes.
It was a heady effort to match his heat win two days ago.
The first five in each semifinal automatically went through to Wednesday morning's final. The two fastest losers would also advance.
The first semifinal was won by impressive Algerian Taoufik Makhloufi in a sluggish 3m 42.24s. Defending champion Asbel Kiprop, Mekonnen Gebremedhin (Ethiopia), Leonel Manzano (USA) and Henrik Ingebrittsen (Norway) also advanced from semifinal one.
Advancing in the second semifinal were winner Abdalaati Iguider (Morocco), Kiplagat (Kenya), Willis, Nixon Chepseba (Kenya) and Matthew Centrowitz (USA).
The two fastest losers came out of Willis' heat. They were Ilham Tanui Ozbilen (Turkey) and Belal Mansoor Ali (Bahrain).
