Geoff Ogilvy is alone in third place at 8-under-par 205 after a third-round 68. Steve Lowery, one of three co-leaders after Friday's second round, posted a 1-under 70 and is tied for fourth with Stuart Appleby, who carded a 67 on Saturday. That duo is knotted at minus-7.
Tiger Woods, the defending champion and three-time winner of the Western Open, was in danger of missing the cut Friday afternoon, but extended his consecutive cuts made streak to 126.
On Saturday, the top player in the world put together a round of 6-under 65 and is tied for sixth at minus-5. Jim Furyk (68), who is playing in only his second tournament since returning from wrist surgery, and Mike Small (69) joined Woods four shots off the lead.
'I have a chance,' said Woods. 'That's what I wanted to do is go out there and play well enough where at least I have a chance going into Sunday, and now if I play a very similar round, like I did today, tomorrow, you never know.'
Woods started on the second nine Saturday and flew out of the gate with three birdies in a row. A poor drive at the par-5 15th cost Woods a bogey but he made the turn at 3 under and got himself back in the tournament.
He tallied three birdies in his first five holes on his second nine but dropped shots at six and seven. Woods recovered with back-to-back birdies at eight and nine to get himself in contention for his first stroke-play victory since last year's WGC-American Express Championship.
'I've been down a few times. It's just part of the sport,' said Woods, who won this year's WGC-Accenture Match Play Championship. 'You've got to keep fighting and keep grinding, and I have plenty of holes left to try and make it up.'
Woods will be trying to catch Ames and Hensby. On Saturday, Ames, from Trinidad, parred his first four holes before reaching the long par-4 fifth. He knocked a 5-iron to 12 feet and holed the putt to get the round going.
Ames sank a 4-footer for birdie at No. 8 but his play on the back nine is what sits him atop the leaderboard. He drained a 20-foot birdie putt at the 10th, then parred his next four.
Ames reached the green with a 3-wood at the par-5 15th, then two-putted for birdie from 20 feet. He kicked in a pair of 4-footers for birdie at 16 and 17 and made it four birdies in a row to close his round when he holed out from a bunker.
Ames owns eight top-10s this season, including seven in his last nine starts. So is PGA Tour victory No. 1 around the corner?
'At this stage I don't press myself to see if I should win or can win,' said Ames. 'At this stage I'm trying to play each day as it comes, and when the chips fall where they should or they shouldn't, I'll just take it from there and go on from there.'
Hensby hit a 6-iron to 15 feet to set up birdie at the fifth. He closed his front nine with back-to-back birdies at eight and nine to stay atop the leaderboard.
Ames went by Hensby into first but Hensby tied him at the par-5 15th. Hensby missed the green with his second shot but chipped to 10 feet and made the birdie putt.
Hensby made some great par saves on his way into the clubhouse. At the 17th, Hensby drove into a bunker, then missed the green short with his second shot. He chipped to 6 feet and rolled home the putt to stay knotted in first.
He missed the green at 18 but nearly holed a shot from 30 yards. Hensby settled for the tap-in par and his first piece of a 54-hole since joining the tour in 2000.
'It was a good day,' said the Australian. 'I struggled a little bit at the end, but I didn't think that really had to do with my golf swing or golf game. It had to do with the wind a little bit. I played tough at the end, but I hung in there and played well.'
Shigeki Maruyama (67), John Rollins (69) and Robert Allenby (71) are tied for ninth place at 4-under-par 209.
Charles Howell III and Matt Gogel were tied with Lowery for first after 36 holes but both struggled on Saturday. They shot matching rounds of 3-over 74 and are part of a group tied for 12th at minus-3.