The two-times world champion felt he short-changed the fans who braved the hazardous winter weather in London to watch what developed into a wretched match between himself and world number one Selby at Alexandra Palace.
Coming hard on the heels of a dismal performance by tournament favourite Judd Trump in his 6-1 drubbing by Graeme Dott in the afternoon, Williams feels that O'Sullivan's crowd-pleasing presence is missed on such days.
O'Sullivan opted out of this year's Masters, an event carrying a new sponsor in Betfair and an increased first prize of £175,000, for the first time in two decades after announcing in November that he was withdrawing from the rest of the season citing personal reasons amid his well-publicised problems battling depression away from the game.
O'Sullivan may return to defend his World Championship in May while Williams tries frantically to regain the form that helped him become world champion in 2000 and 2003.
"A few others are struggling up here, but my performance was just embarrassing," said Williams, whose best break against Selby was a paltry 48.
"It was a pity to watch, really. Fingers crossed Ronnie O'Sullivan comes out of retirement and starts playing again."
Williams, who won the Masters in 1998 and 2003, continued: "I missed so many easy balls. It was a shocking performance. That out there tonight was the worst I have played as a professional, I have to say.
"I don't know if it is a confidence thing. If I knew I'd tell you. You tell me? Selby played rubbish and beat me 6-1. "It was just embarrassing to play like that."
Williams threw his cue to the ground in frustration at one stage.
"It wasn't nasty or anything like that," said Williams, who also dispelled suggestions he could quit due to his poor form.
"I felt like throwing it into the crowd or snapping it over my knee. It is just frustrating playing that poorly."
Selby will face Graeme Dott in the semi-finals as he continues his quest for a third Masters title on Saturday night, but was not pleased about his own contribution to the evening.
"I've got a lot of respect for Mark. He played nowhere near how he can perform. We all know that," said Selby. "Mark missed a lot of balls I didn't expect him to miss so I lost focus and it became a strange game.
"You need to put friendships aside and focus on the job, but it became very difficult and I probably lost focus from the third or fourth frame. I don't think I won that match. Mark just gifted me a lot of the frames, and I picked up the pieces. I'm delighted to be through, obviously.
"I'm not surprised Graeme (Dott) is through to the semis. He is a class player, and has been world champion."
Selby claimed the opening four frames against Williams, who blew obvious chances to win at least three of them.
The Welshman led by 25 points with 25 points remaining in the first frame.
But Selby unearthed the four points he needed to steal it as Williams failed to escape from a snooker before his opponent cleared up to pinch the frame.
Williams was again found wanting in the second frame when he missed a red to a centre pocket. It was a horrid miss and was duly punished by Selby in moving two frames ahead.
The spirit seemed to be visibly draining from Williams as a 48 in the third frame failed to pay dividends as another bad miss enabled Selby to polish off from the green to snatch the frame by 10 points for a 3-0 lead.
Williams compiled 27 in the fourth frame before another demoralising error enabled Selby to make 48 for a 4-0 advantage at the mid-session interval.
Williams ran in 39 in the fifth frame before jawing a blue to a centre pocket, but he needed to bag a fine late green in the frame to finally provide himself with the platform for his solitary frame of the night in trailing 4-1.
Williams paid a heavy price for missing a cut on a red early in the next frame as Selby compiled 73 to move 5-1 ahead which quickly became 6-1.