Hansen finished with an 11-under 133 total and Westwood had four bogeys in the final eight holes for a 73 in windy conditions. Westwood shot a 61 Thursday, tying his career best.
Ahead by four strokes most of the round, the Ryder Cup player bounced his 16th-hole tee shot 100 feet past the pin on the 195-yard par-3. On the 18th, Westwood's approach shot dropped into the water.
'I didn't think I played as badly as a 73 to be honest,' said Westwood, chasing his second win this year. 'It was just one of those days. The wind got up towards the end of the day.'
Hansen, looking for his first European Tour win since the 2002 Irish Open, scattered five birdies and a bogey.
Bernhard Langer (68) shared third with Simon Khan (70) and Simon Dyson (71), four strokes back. Colin Montgomerie (69), Paul Lawrie (70) and John Daly (71) were among those another stroke back.