The world No 4's scintillating 11-under-par 61 helped him take a share of the lead, while overnight pacesetter, South African Louis Oosthuizen, added a late touch of drama when he missed a chance to go back out in front on the last.
Beyond the leading duo, who are locked together on 18 under, Brandt Snedeker went within inches of achieving golf's magical 59 when he missed a 15-footer for birdie on the final hole, with the American instead having to be satisfied with a course-record 60 – a shot better off than Westwood.
But although Snedeker shot the lowest, Westwood was the day's biggest winner.
The 39-year-old Englishman had an unblemished third round of 11 birdies and soared up the leaderboard from 12th to joint first, with himself and Oosthuizen, the 2010 Open champion, three shots clear of nearest challenger, Phil Mickelson.
The Ryder Cup star set his stall out by birdieing the first three holes and he did not look back.
His putting was in good fettle – he landed a tricky 10-footer on the ninth to emphasise such – and he looked in control all the way round.
Oosthuizen had led by five going into the day, but a sluggish display with the putter cost him dear.
He had several near misses on his way around – showing signs of decline when he slid one wide from five feet – and things were summed up by his penultimate shot of the day when a 15-footer on the last would have given him a one-shot lead heading into tomorrow. He missed by an inch.
The same fate befell Snedeker, who looked set to go sub-60 when he approached the 18th, only to pull up just short.
Earlier in the day, Ernie Els had briefly snagged a share of the lead on his way back to the clubhouse, but undid his hard work when he found the water on the 15th, taking a double bogey. He finished with a 69 and slept on 14-under, along with Bill Haas and Ian Poulter.
Snedeker is a shot further back, but seemingly the man with the momentum to mount a last-day assault. His overnight mood could have been even better too. He said: "I was disappointed not to shoot 59.
"In a career you might only get a couple of chances to do it. When I made eagle at the 15th the idea of shooting 59 came to me, but I knew I needed to make birdie at the last three holes."
Indian Gaganjeet Bhullar is a shot further back after an impressive nine-under round, with Martin Kaymer, Carl Pettersson, Scott Piercy and Adam Scott alongside him on 12- under overall.
In a highly competitive field, Luke Donald and Jason Dufner are next on 11-under, although it was not such a good day for everyone, as Dustin Johnson slipped from nine under to three over thanks to a horror 12-over round, with a quadruple-bogey nine on the 15th the low point.
o Thailand's Numa Gulyanamitta marched to the top of the leaderboard the Sanya Ladies Open in China after carding five birdie and two bogeys at Yalong Bay Golf Club yesterday.
The 23-year-old LPGA rookie's round of three-under 69 took her to five under par in total and one clear of Cassandra Kirkland of France.