Australian Adam Scott held off a bold early challenge from Japanese teenager Ryo Ishikawa to clinch his eighth PGA Tour victory by four shots at the WGC-Bridgestone Invitational on Sunday.

One ahead overnight, Scott tightened his grip on a maiden World Golf Championships (WGC) title with three birdies in five holes after the turn before closing with a flawless five-under-par 65 to match the day's lowest score.

With Tiger Woods's former caddie Steve Williams on his bag for a fourth tournament in a row, Scott kept his cool on a hot and humid afternoon at Firestone Country Club to post a 17-under total of 263.

The 31-year-old Australian finished in style, sinking a five-foot birdie putt on the 18th green and pumping his fist in triumph before being warmly embraced by Williams.

"Today it was on, I stayed so patient," a beaming Scott said greenside. "It was a good round of golf. To win here at this place, and a World Golf Championships title, it's huge."

Asked how much he had benefited from having Williams on his bag, Scott replied: "Are you kidding me? He knows this place better than anyone."

Williams, fired by Woods on July 3 after a highly successful relationship dating back to 1999, told CBS television: "I've been caddying for 33 years, 145 wins, and that's the best week of my life."

Ishikawa, at 19 bidding to become the youngest PGA Tour winner in 100 years, bogeyed the final hole for a 69 to finish tied fourth at 12 under with Australian Jason Day (69).

British world number one Luke Donald and American Rickie Fowler each birdied the 18th for matching 66s to share second place at 13 under.

Woods, a seven-times champion at Firestone with Williams on his bag, ended an erratic week with a level-par 70 to finish joint 37th, 18 shots off the pace after his first tournament start in three months.

HEAD-TO-HEAD AT FIRESTONE

For much of an intriguing afternoon the tournament came down to a head-to-head between Scott and Ishikawa before the 19-year-old's challenge faded over the closing stretch.

Ishikawa came close to eagling the par-five second after unleashing an exquisite three-iron to within five feet of the cup. However his first putt slid past and he had to settle for a tap-in birdie.

That left him at 12 under, and a stroke behind Scott who also birdied the hole after getting up and down from a greenside bunker.

Ishikawa closed the gap at the par-three fourth where he nervelessly rolled in a 15-foot birdie putt to join Scott at the top.

The Japanese briefly slipped back with a bogey at the fourth after he drove into the left rough but he recovered with a birdie at the tricky par-four sixth where he drained a 21-footer.

Ishikawa again faltered with a bogey at the eighth but went on to trade birdies with Scott at the par-four 10th to remain a stroke behind.

The Australian doubled his lead at the par-three 12th where a chip-in from the fringe of the green put him at 15 under.

Scott forged three strokes clear by rolling in a curling 28-footer to birdie the 14th before pumping his right fist in celebration.

Though he failed to birdie the 16th, he picked up his fifth shot at the last to become the second Australian to win the WGC-Bridgestone Invitational, following Craig Parry in 2002.