Early-starting Rory Sabbatini set a target for the rest of the field to chase at the WGC Bridgestone Invitational with a blistering finish to his second round in Akron, Ohio.

South Africa's Sabbatini shared the one-shot overnight lead with England's Paul Casey and American Hunter Mahan on three under par at the Firestone Country Club's testing par-70 South Course.

But while Casey was one of the many to struggle in difficult early conditions, carding a three-over round of 73, Sabbatini recorded a second consecutive 67.

Ireland's Padraig Harrington carded an opening two over par round yesterday but has improved to level after a brace of early birdies on his second round.  The Dubliner is through five holes.

Darren Clarke is also level par for the tournament through six holes of his second round while Paul McGinley is two over for his round after eight, four over in total.

Mahan was among the afternoon starters and Sabbatini was one of the handful of players out on the course who was actually improving his score in humid, 85-degree heat.

The South African made the turn in 34, with birdies at the first and fifth holes before a bogey at the sixth.

Another dropped shot stalled Sabbatini at the 14th but he bounced straight back with birdies at 15, 16 and 17.

That left him at six under for the tournament, two shots ahead of Masters champion Zach Johnson, who shot the lowest round of the day so far with a five-under 65.

Boo Weekley joined Johnson on four under with an eagle three at the par-five second.

Casey, though, had a day-long battle to stay in touch and finished the day at level par for the tournament, six shots adrift of Sabbatini.

Three bogeys and three birdies on the front nine set the trend and a birdie at 11th was followed by a bogey five at the 13th before a birdie at the par-three 15th sent him back to three under.

Then it fell apart again for the Ryder Cup star as Casey bogeyed the 16th and double-bogeyed the 18th for a disappointing end to a tough day.

Lee Westwood had an inauspicious start to his second round, bogeying his first two holes having teed off at the 10th hole.  But he steadied himself with a run of six pars before birdies at the 18th, first and third took him to within a shot of Sabbatini, only to card bogeys at the fourth and sixth to end the day with a 71 at one under for the tournament.

Playing partner Sergio Garcia, however, was less successful. The Spaniard, playing his first tournament since his blowing his third-round lead at the Open at Carnoustie two weeks ago, started the day at one over par and birdied his first hole at the 10th.

A bogey at the 16th, though, was followed by a quadruple bogey eight at the 18th and Garcia began his inward nine with a run of bogey, birdie, birdie, bogey before a double bogey at the sixth left him at eight over for the tournament with a round of 77.