Open champion Padraig Harrington returned to action for the first time since his maiden major victory at Carnoustie a fortnight ago and carded a two-over-par 72 at the WGC Bridgestone Invitational in Akron, Ohio.

Harrington, playing with Boo Weekley and Ryder Cup team-mate Darren Clarke, began brightly with a birdie at his third hole, Firestone's 12th, but the Dubliner followed with three bogeys.

Clarke finished on even par after four birdies were book-ended by four bogeys.

He had begun in terrible fashion, bogeying the 10th, 13th and 16th before getting back on track with successive birdies at the 18th and first holes and two more at the fourth and seventh before being pegged back with a finishing bogey at the ninth.

Colin Montgomerie was left feeling confused following a "telephone number" back nine that still left him within striking distance of the first-round lead.

Montgomerie carded a one-over par 71 at Firestone Country Club which put him just three strokes back from early leaders Tiger Woods, Mark Calcavecchia, Stuart Appleby and Weekley.

Hunter Mahan later recorded an opening 67 to take an outright lead among the morning starters.

But Montgomerie's one eagle, two-birdie, two-par, four-bogey back nine scores of five, six, two, five, six, two, five, two, four left the Scotsman bewildered.

"You've read my card on the back," Montgomerie said. "It's a telephone number. A good round, I suppose. Don't ask me how.

"When two (under) is leading one over is okay. There's nothing wrong with that, we'll see how we go. It's not easy out there."

The highlight of Montgomerie's round, which also included two birdies on the front nine, was his eagle on the par-four 17th, when he holed out with a seven iron from 156 yards.

The Scot added: "I've got lots of twos over the years. There are thousands and thousands of twos, not too many eagles though."

An eagle three at the par-five second hole gave England's Justin Rose a boost which helped him to a one-under round of 69 on the par-70, 7,455-yard South Course.

Starting on the 10th tee, Rose had shared the lead on two under with Australian Appleby before a bogey four from home sent him back into the pack.

Defending champion Woods, seeking a second hat-trick of consecutive wins in the event, opened with a two-under 68, to join Appleby and Calcavecchia in the morning lead - and they were quickly matched by American Weekley.

World number two Jim Furyk withdrew from the tournament due to a back injury before the first round got under way.