Phil Mickelson's bid to win his first US Open will go down to the wire, says Colin Montgomerie.
Mickelson, a five-time runner-up, is in pole position to win the title on his 43rd birthday after finishing the third round with a one-shot lead over Hunter Mahan, Charl Schwartzel and Steve Stricker.
Sky Sports pundit Montgomerie - himself a three-time US Open runner-up - believes the American will be crowned champion if he can shoot a final round of two-under but stressed that the difficulty of the closing holes at Merion mean that any one of the leading 15 players could lift the trophy.
"I think anyone within six shots has a chance - so that includes Ian Poulter, even though he finished poorly," reflected Montgomerie.
"Rickie Fowler went out and shot 67, so you never know what might happen. Fowler has moved way up from just making the cut to being almost in the top 10.
"It's amazing what can happen because this finish is most difficult. People can drop shots on 17 and 18 - they can do anything.
"Luke Donald made a bogey and then had a six at 18, which was disappointing for him but he's still in there, as is Justin Rose. But Phil Mickelson is very much the favourite now and we would all love him to win."
Just five shots separate the top 10 players, but of that group only Mickelson - who has never held the outright lead in the US Open going into the final day - and Schwartzel have experience of winning majors.
After so many close misses, Montgomerie believes that Mickelson must grab this chance for fear that another one might not come his way.
"Sunday is a big day for him. At 43 age is against him - this could be his chance to win the national championship in America and we wish him well.
"I think he has played the best of anyone out there. He is leading and he's the only one under par. But if the weather is the same as it is, he needs to shoot 69.
"Being British, I'd like to see Luke Donald, Justin Rose or Ian Poulter winning but wouldn't it be nice to see Mickelson finally win."
Mickelson has never won the US Open, finishing runner-up five times - but this will be the first time he's held the outright lead going into the final day."
After finishing his third round on Saturday - an even-par 70 that leaves him on a one-under-par tally of 209 - Mickelson described playing Merion as "a hard challenge" but admitted "it is a lot of fun".
Sky Sports pundit Butch Harmon, who is Mickelson's coach, believes his charge is mentally prepared to complete the job.
"Just like yesterday, Mickelson got off to a shaky start - he wasn't in control of his golf swing and was two-over early on," said Harmon.
"The thing that he said that I like the most is he said 'I have to have patience - I couldn't force the issue because the golf course is too hard; I had to take advantage of some of the easier holes'.
"He made those birdies at 10 and 11 to get himself up there and the birdie at 17 was absolutely phenomenal - just unbelievable. He had a bit of bad luck at 18 but that's golf. He's in good position.
"He also said 'I have to shoot under par in the final round to win', so he's geared up for the challenge.
"This rough is brutal and that's the reason there is only one person under par. There is a premium on hitting the fairways and hitting the greens.
"You've got to play this golf course from the short grass. It's the only way you can shoot a good score."
Watch live coverage of the fourth and final round of the US Open from Merion on Sky Sports 2 HD from 5pm on Sunday.