Phil Mickelson said he still felt unhappy with his game despite improving five shots on his opening round by carding a 69 on day two of the US Open at Congressional.

The left-hander actually had one of the best rounds of the day going after playing his first 17 holes in four-under to climb to one-under for the tournament following his poor first round 74.

But a double bogey at 18 after a wild tee-shot left and a second which ran into the water knocked him back to one-over after 36 holes - 12 off the lead - and left him reflecting on a disappointing opening two days.

"Yeah, it was a disappointing finish with that double, but I'm still struggling," said the four-time major winner and five-time US Open runner-up.

"Even though I was able to shoot under par today [Friday], I was still struggling with it. Rather than go work on it, I think I'm going to try to figure out what it is I need to work on first and then get back to the range."

While Mickelson has struggled, his playing partner Rory McIlroy has looked imperious and the left-hander was quick to offer praise to the 22-year-old.

"He's striking it flawlessly and putted great on the greens. His first two rounds were very impressive."

Steve Stricker, another fancied American, also improved on his poor opening round, adding a two-under 69 to his day one 75 to finish at two-over.

That left him comfortably inside the cut line and he said: "Today [Friday] was a better day. Could have been better. I didn't make some putts today [Friday] that I normally make or should make, so in that regard a little bittersweet. But all in all, I'll be playing the weekend, so it's a good thing."

As for chasing down McIlroy, Stricker reflected: "Just got to keep hanging in there and keep trying to shoot under par. He's got to come back. The way he's playing now, it doesn't seem like he'll do that, but you've just got to keep fighting and see what happens. But pretty incredible what he's done so far.