I don't think you can really, really count that as a tune-up. Williams was the mandatory challenger for Margarito's title.
This really wasn't a tuneup fight. Jones vs. Lacy was actually a tuneup fight for Jones vs. Geen. Jones didn't even agree to fight Hopkins until well after he agreed to face Green. They're still fighting eachother regardless.
Ray Mercer was in line to face Riddick Bowe in the 90's. Jesse Ferguson decisioned him and got the shot instead.
hopkins v tito. tito and roy jones was meant to be the one everyone was hyping for. both were practicaly undefeated and being compared to sugar ray robinson- while hopkins was as underappreciated as they come. makes u put in perspective how we think of pacquiao and mayweather. theyre all time greats now- but a few bad losses- and people change their minds real quick.