An impressive back-nine performance has seen Phil Mickelson claim a one-shot lead heading into the final round of the US Open with Australia&aposs Jason Day still in touch. Mickelson notched three birdies on the back nine and despite a bogey on the par-four 18th hole, finished the day one-shot clear at the top of the leaderboard, with Hunter Mahan, Charl Schwartzel and Steve Stricker tied for second. Day matched Mickelson&aposs three birdies and a bogey on the run home to finish three shots adrift of the American after a two-under 68. Having finished second at the US Open on five occasions, Mickelson completed Saturday&aposs third round on one under overall after an even-par 70. Mickelson&aposs playing partner Luke Donald of England endured a disappointing end to the third round at Merion Golf Club in Pennsylvania, bogeying the 17th hole and then struggling in the rough on the 18th to post a double-bogey and slip two strokes adrift of the 42-year-old leader. Donald is tied for fifth at one over with compatriot Justin Rose and Billy Horschel with Day one stroke further adrift. Saturday&aposs play began with those who had not played two full rounds doing so, with a series of players vying to make the cut. The cut was called at eight over par, with five former US Open champions Graeme McDowell (+13), Michael Campbell (+14), Angel Cabrera (+15), Lucas Glover (+16) and Jim Furyk (+16) missing out on playing the final two rounds. Mickelson and Horschel shared the lead at the start of the third round but Schwartzel stormed up the leaderboard early with consecutive birdies on the third and fourth holes to put pressure on the leading pair. Both Mickelson and Horschel bogeyed the third and fifth holes to drop out of the lead, as Donald, Rose and Australia&aposs Josh Senden took over at the top. But the back nine brought the field back together with Mickelson birdying the 10th, 11th and 17th holes to lead the US Open with a day to play, while Mahan carded four birdies and two bogeys on his way to a one-under 69 to remain within touching distance of top spot. Schwartzel seemed to stall on the back nine with a birdie on the par-four 10th hole followed by bogeys on the 17th and 18th, while Stricker birdied the 10th and 12th to join the South African and Mahan on even par overall. Donald would have finished out in front but for a demoralizing end to his third round, bogeying the 17th before seeing his second shot on the 18th sink deep in the rough, eventually leading to a double-bogey. Senden plummeted down the leaderboard on the back nine with two bogeys and a double-bogey to end the day on five-over for the tournament.