The Ospreys wing was a second half replacement in the 28-25 last gasp defeat in Pretoria - a defeat he admits left the Lions shattered.
"It has taken two days for the boys to get over it, both physically and mentally. There was a safari trip organized for Sunday and Monday but a lot of the guys, including myself, who wanted to go didn't because we were simply exhausted from the match," Williams revealed in his Daily Telegraph blog..
"Losing this series is one of the lowest points in my career and this has certainly been the toughest tour I have ever been on. We felt we deserved something from both Tests and probably a fair reflection would have been 1-1 instead of 2-0.
"Yet despite our disappointment, we feel we have restored the credibility of the Lions by pushing the world champions so close. Either Test could easily have gone the other way and the boys know they did everything they could.
"We would like to think we haven't let anybody down. We certainly haven't let ourselves down and are proud of what we have done. And that makes us even more determined to win the third Test on Saturday.
"We are struggling with a few injuries, and South Africa will be confident of finishing us off. But we want to go out on a high. All we can do now is go out and play."