American Hunter Mahan eagled his penultimate hole to grab a  share of the halfway lead at the WGC-Cadillac Championship on Friday as the Blue Monster showed its true colours, with more than 100 golf balls ending up in the water.

On a gruelling day that some players described as a survival test, Mahan eagled the par five eighth in fading light for a second round of two-over 74 that left him tied for the lead with his compatriots Dustin Johnson (74), Patrick Reed (75) and Matt Kuchar (74) at one-under par.

Northern Irishmen Rory McIlroy (74) and Graeme McDowell (71), Italian Francesco Molinari (75) and Welshman Jamie Donaldson, who posted the low round of the day with a two-under 70, were all lurking one shot further back.

"I felt stressed all day because I knew every shot had a big penalty written all over it, staring at bogey or worse I thought at a well struck shot," Mahan told reporters. "It was a really tough day.

"There wasn't an easy shot out there and you know, you're going to have to find a way to make some putts.

"One of those rounds where it could go south pretty fast so you've got to grind it out and find a way to get a number up there and get to the weekend."

Long regarded as one of the PGA Tour's great layouts, the Blue Monster course underwent a massive renovation that made it even more frightening and almost unplayable.