Phil Mickelson has insisted that his persistence on the greens has kept the American's chances of a maiden US Open title firmly alive.

Mickelson is joint-leader at the Merion Golf Club with Billy Horschel after carding a second-round score of 72, with the two frontrunners currently the only players under par in the tournament.

The 42-year-old admitted that he let chances go to improve his standing slip on a difficult second day, but believes that he is striking the ball cleanly and showing fighting spirit at the halfway stage of an intriguing tournament.

After making a fine birdie putt at the 18th hole, Mickelson told Sky Sports News: "I left it in a good spot, that was a really tough shot. I was trying to make par, I certainly wasn't aiming for birdie up there.

"I had many opportunities on holes I should be able to birdie but didn't capitalise, but I played the hard holes extremely well and made pars I played the hard holes extremely well but I wasn't able to make birdies. I fought hard and kept making solid pars.

"I've been striking the ball really well, but to let a couple go today on the greens was not consistent with how I've been playing. I fought hard, hung in there, even though the score wasn't what I wanted."

Just three shots separate the top 12 players after a marathon second day at the US Open, with a large section of the field unable to complete their second rounds after poor light stopped play late into the evening.