Pebble Beach, California - Wind could add to the vexing mix at Pebble Beach on Saturday when US Open leader Graeme McDowell of Britain and challengers including Phil Mickelson and Ernie Els set off in the third round.
Small, fast greens, heavy rough and ocean cliffs have been enough to perplex a field that produced only five players under par through 36 holes. A forecast of winds approaching 20 miles an hour (32-kph) would make the test that much more difficult.
Masters champion Mickelson, aiming to win his first Open after five runner-up finishes, expected conditions to get harder in the final two rounds.
"Typically the weekend of a U.S. Open the greens get firmer, the pin placements get tougher, the rough usually is not cut so it grows a little bit longer," he said after a dazzling 66 on Friday lifted him within two shots of McDowell.
"It just gets more challenging."
Heavyweight contenders such as world number one Tiger Woods and Padraig Harrington might have a window of opportunity teeing off about two hours earlier than the leaders with winds expected to increase as the day progressed.
Northern Ireland's McDowell, looking for his first major triumph and seeking to end a 40-year victory drought for Europeans at the US Open, held a two-stroke lead after a 68 put him on three-under-par 139.
McDowell was set to tee off at 15.50 (22.50 GMT) with long-hitting American Dustin Johnson, winner of the last two US PGA Tour events held at Pebble Beach.
The penultimate pairing featured double U.S. Open winner Els of South Africa and 18-year-old Japanese sensation Ryo Ishikawa, both on 141, with four-times major winner Mickelson (141) and Germany's Alex Cejka paired just ahead of them.
Left-hander Mickelson, who celebrated his 40th birthday this week, made his move on Friday with a confident putter after tweaking his putting alignment following an opening 75. - Reuters