South African Els, a three-time Major winner, briefly moved into a share of second place during Sunday's final round before closing with a two-over-par 72 in difficult scoring conditions to finish three shots behind champion Webb Simpson.
"I'm contending again ... and that means a lot," Els wrote on his official website. "I feel I have a chance now and that I could win another one of these Majors. I've got my belief back.
"Right after finishing my final round at Olympic I was disappointed to have let slip a chance to win my third US Open, but having had a night to sleep on it I can reflect more clearly on the positive aspects of my performance.
"My mental attitude was good all week and I played really well on what was a very tough golf course. I had a couple of train wrecks, on holes four and 16, and ultimately I guess they cost me the tournament."
Els, whose last Major victory came a decade ago at The Open, recorded a double-bogey at the fourth and a triple-bogey at the 16th in the opening round at Olympic.
After making three early bogeys in the third round, the smooth-swinging 42-year-old roared into contention with three birdies and an unexpected chip-in for eagle at the par-five 17th.
In the final round, he rolled in a curling 20-foot eagle putt at the driveable par-four seventh to move briefly into a tie for second but bogeys on the next two holes derailed his title bid.
"When I look at where I was this time last year and where I am now it's a huge change for the better," said Els, a twice US Open champion in the mid 1990s.
"The whole week at Olympic Club I was a lot calmer and I felt much more in control of my game and my emotions. That's a really good sign going forwards."
Els, who won the most recent of his 18 US PGA Tour titles at the 2010 Arnold Palmer Invitational, had slipped to 56th in the world rankings by the end of last year.
Boosted by his top-10 finish at the US Open on Sunday, he has risen to 39th.