Rory McIlroy has admitted it will require a "heroic effort" to win the BMW Championship despite surging into a share of second place with a superb third round in St Louis.
The Northern Irishman produced a five-under-par 65 at Bellerive Country Club to move to 12 under for the tournament and keep his hopes of victory alive heading into Sunday's final round.
But McIlroy faces a formidable task after Wyndham Clark strengthened his grip on the tournament by opening up a five-shot advantage at the top of the leaderboard.
Clark also carded a 65 to reach 17 under, leaving McIlroy and Patrick Cantlay, who shot 66, as his nearest challengers.
McIlroy had surrendered his first-round lead following a 69 on Friday but responded impressively on Saturday, particularly during a blistering finish.
The four-time major champion birdied four of his final six holes to propel himself back into contention despite struggling badly for accuracy from the tee earlier in his round.
McIlroy remarkably failed to find a fairway until the 10th hole but relied on his short game to prevent his challenge from falling apart.
"I needed to be patient the last three days, because my game has been good in parts, but then it has been a struggle in other ways," McIlroy said.
"I didn't hit my first fairway today until the 10th hole.
"Playing this golf course from the rough isn't fun, but I hung in there, my short game saved me a couple of times and I got some momentum coming in."
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Despite his encouraging finish, McIlroy conceded something extraordinary may now be required to overhaul Clark.
"I am in a good spot in terms of where I am at on the leaderboard, but it's going to take a heroic effort from one of us to catch Wyndham," he added.
Clark's five-shot advantage appeared unlikely after a disastrous opening hole saw the American make double bogey.
However, he responded emphatically.
Two birdies steadied his round before an eagle at the eighth sent him through the turn at two under for the day. Clark then birdied four of his next five holes, with only a bogey at the last preventing an even bigger advantage.
Chris Gotterup and Gary Woodland remain within striking distance on 11 under, while Collin Morikawa sits another shot back on 10 under.
South Korea's Im Sung-Jae is eight shots behind Clark on nine under.
Further down the leaderboard, world No 1 Scottie Scheffler and Scotland's Robert MacIntyre both shot 68 to join a group of eight players on five under.
English duo Aaron Rai and Matthew Fitzpatrick are one stroke further back.
It proved a more difficult Saturday for Justin Rose, whose 73 dropped him to one under, while Tommy Fleetwood endured a 74 to fall to two over.
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