|Dell Championship third-round leaderboard|
|-12 J Thomas (US), M Leishman (Aus); -11 P Casey (Eng); -10 J Spieth (US), G Murray (US), A Hadwin (Can); -9 D Johnson (US), J Rahm (Sp); -8 P Mickelson (US), P Cantlay (US)|
|Selected others: -7 J Rose (Eng); level H Matsuyama (Jpn), I Poulter (Eng); +1 J Day (Aus), M Laird (Sco); +2 S Garcia (Sp); +3 R Knox (Sco)|
US PGA champion Justin Thomas produced a superb eight-under 63 to tie for the lead after the penultimate round of the Dell Championship in Massachusetts.
Despite dull, soggy conditions at TPC Boston, the 24-year-old world number seven recorded 12 threes, a record for a round in the FedEx Cup play-offs.
He was joined at 12 under by Australian Marc Leishman, with Englishman Paul Casey one back after a 67.
Overnight leader Jon Rahm birdied the last for a 71 and remained nine under.
Casey, 40, has won only once on the PGA Tour, in a play-off at the 2009 Houston Open, but he dropped only one shot in the rain and made five birdies.
Open champion Jordan Spieth had six birdies in a 66 and is just two off the lead. World number one Dustin Johnson birdied four of the last five holes in a 66 to be only one further back.
Thomas has now gone 49 holes without a bogey, with his only dropped shot in the tournament coming at the fifth on day one.
In January the American became the youngest man to break 60 on the PGA Tour when he carded an 11-under 59 at the Sony Open in Hawaii, while in June he was the first man to shoot a nine-under-par round at the US Open.
“You can’t really compare but in terms of ball-striking that was right up there. I drove it better than I have all year,” he said.
Rahm began in assured fashion with a birdie from eight feet at the first but briefly lost the lead after dropping a shot at the next when his approach found the water hazard. His closing two-putt birdie at the par-five 18th was the only shot he gained in his final 11 holes.
American Kyle Stanley led after three birdies in his first four holes but three shots went in the next two and he finished in a tie for 19th at five under.
Garcia forced to make putter change
Masters champion Sergio Garcia, who began round three on two under, smashed his putter against a sprinkler head in exasperation at the fourth hole and opted to putt with his three-wood.
He holed his first attempt with it from 14 feet for a birdie but he then switched to a driver and took 12 putts in the next five holes.
The Spaniard then switched to putting with a three-iron but his unhappy day finished with a double bogey and a 75 which left him at two over for the tournament.
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