Thomas Pieters opened up a two-shot lead as the third round of the WGC-Bridgestone Invitational moved towards its conclusion at Firestone Country Club.
The big-hitting Belgian started the day two shots off the lead but turned in 31 with the help of a birdie-birdie-birdie start to get to nine under and lead the way from Australian Scott Hend.
Scotsman Russell Knox, World Number Three Hideki Matsuyama and Americans Zach Johnson and Charley Hoffman were then at six under.
Pieters was playing in just his sixth World Golf Championships event but had never finished lower than 30th with a best result of tied for fifth at the WGC-Mexico Championship earlier this year.
He put his approach to the first to eight feet to get to six under and with playing partner and overnight leader Jimmy Walker finding sand off the tee to surrender a bogey, the Belgian was in a tie for lead.
Knox had a birdie-birdie start and he moved into a share of the lead with the help of an approach to three feet on the fourth but was not there for long as Pieters played a remarkable second after a poor tee-shot on the second and got down in two for a birdie.
A perfect tee-shot on the next then allowed Pieters to put an approach to tap-in range and he led by two.
Another excellent shot from the peg, this time to six feet on the par three fifth, extended the lead to three but the chasing pack would not be shaken off.
Johnson had holed from 18 feet on the first and when some stunning iron play saw him make gains on the fifth and sixth, he had cut the gap to two. The 2015 Open champion bogeyed the next but Matsuyama, Hend and Knox soon all made further moves.
Matsuyama had birdies on the second and fifth and when he rolled in a 25-footer on the eighth, he was up to seven under.
He was soon joined by Hend who had birdies on the first, fifth, sixth and eighth to go with a bogey on the seventh to turn in 32. He added further gains on the tenth and 14th and a brilliant tee-shot on the par three 15th moved him two off the lead.
Knox had made four pars in a row but birdied the ninth and when Johnson bounced back with a gain on the eighth courtesy of a 25-footer, four players shared second place.
That was soon three off the lead, though, as a wonderful second into the eighth left Pieters with nine feet to get to ten under.
Knox and Johnson then gave shots back on the tenth and ninth respectively and with Matsuyama also surrendering a stroke in the tenth, Pieters' position was strengthened further.
He was staring a double-bogey in the face after a poor tee-shot and chip on the ninth but he holed a clutch putt and held a two-shot advantage.
Hoffman was going about his business with the minimum of fuss and had gains on the second, sixth and tenth,
World Number Four Rory McIlroy, Australian Jason Day and Canada's Adam Hadwin were at five under, a shot clear of Race to Dubai Rankings presented by Rolex leader Tommy Fleetwood and Open champion Jordan Spieth.
Scotsman Russell Knox, World Number Three Hideki Matsuyama and Americans Zach Johnson and Charley Hoffman were then at six under.
Pieters was playing in just his sixth World Golf Championships event but had never finished lower than 30th with a best result of tied for fifth at the WGC-Mexico Championship earlier this year.
He put his approach to the first to eight feet to get to six under and with playing partner and overnight leader Jimmy Walker finding sand off the tee to surrender a bogey, the Belgian was in a tie for lead.
Knox had a birdie-birdie start and he moved into a share of the lead with the help of an approach to three feet on the fourth but was not there for long as Pieters played a remarkable second after a poor tee-shot on the second and got down in two for a birdie.
A perfect tee-shot on the next then allowed Pieters to put an approach to tap-in range and he led by two.
Another excellent shot from the peg, this time to six feet on the par three fifth, extended the lead to three but the chasing pack would not be shaken off.
Johnson had holed from 18 feet on the first and when some stunning iron play saw him make gains on the fifth and sixth, he had cut the gap to two. The 2015 Open champion bogeyed the next but Matsuyama, Hend and Knox soon all made further moves.
Matsuyama had birdies on the second and fifth and when he rolled in a 25-footer on the eighth, he was up to seven under.
He was soon joined by Hend who had birdies on the first, fifth, sixth and eighth to go with a bogey on the seventh to turn in 32. He added further gains on the tenth and 14th and a brilliant tee-shot on the par three 15th moved him two off the lead.
Knox had made four pars in a row but birdied the ninth and when Johnson bounced back with a gain on the eighth courtesy of a 25-footer, four players shared second place.
That was soon three off the lead, though, as a wonderful second into the eighth left Pieters with nine feet to get to ten under.
Knox and Johnson then gave shots back on the tenth and ninth respectively and with Matsuyama also surrendering a stroke in the tenth, Pieters' position was strengthened further.
He was staring a double-bogey in the face after a poor tee-shot and chip on the ninth but he holed a clutch putt and held a two-shot advantage.
Hoffman was going about his business with the minimum of fuss and had gains on the second, sixth and tenth,
World Number Four Rory McIlroy, Australian Jason Day and Canada's Adam Hadwin were at five under, a shot clear of Race to Dubai Rankings presented by Rolex leader Tommy Fleetwood and Open champion Jordan Spieth.
