Overnight leader Westwood was joined atop the leaderboard at five-under par by Sweden's Peter Hanson, who recovered from a double-bogey on the first to make birdies on each of the next three holes.

World number two McIlroy started the day four shots off the pace but the Northern Irishman made three birdies on his first seven holes, including a 35-foot birdie putt on the par-three fourth, to put some pressure on the leaders on the back nine.

Spain's Sergio Garcia also put himself in the frame with a four birdies on his opening eight holes to move within a stroke of the leaders on four-under par.

South Africa's Louis Oosthuizen, who began the day tied for second, was the first of the leading players on course and he had a rough start to his second round with a double-bogey seven on the second hole and was two-over for the day after 14 holes.

Rain was forecast for Friday and temperature dropped sharply overnight to 55 degrees Fahrenheit (13 Celsius), leaving some spectators at Augusta National wrapped in blankets.

The weather was expected to return to sunshine for the weekend of the first major of the season.

Four-times Masters champion Tiger Woods and world No.1 Luke Donald were among the late groups and had yet to tee off.