Queensland-born Jason Day has credited caddie Steve Williams' relentless work ethic for his lowest score in two years on the US PGA Tour after launching himself into weekend contention at the Travelers Championship.
Former world No.1 Day opened the event in Connecticut with a lacklustre, even-par 70 on Thursday (Friday AEST) before accomplished New Zealand caddie Williams demanded a practice session after his first round.
"Pretty much when (Williams) asks you to do something, you do it," Day said.
It paid dividends in Friday's (Saturday AEST) second round, when Day fired a bogey-free, seven-under-par 63 to sit four shots behind the lead.
Unheralded American Zack Sucher (65) took the 36-hole lead at 11-under, with Chez Reavie and Keegan Bradley (both 66) sharing second at nine-under.
Teeing off the 10th hole on Friday with an even-par total, Day jumped out of the gates with five front nine birdies - including four consecutive from the par-three 11th.
The Australian added two more birdies on the front nine and signed for a 63 - Day's lowest round on the PGA Tour since May 2017.
Day put on a ball-striking clinic despite heavy rain lashing the morning groups at the Cromwell, Connecticut course.
He found 11 of 14 fairways from the tee, while hitting 14 of 18 greens in regulation.
Day says there is no doubt his improved swing is due to Williams' insistence the Queenslander practice after every round since they teamed up before last week's US Open at Pebble Beach.
The 31-year-old is buying into Williams' philosophy given he caddied for Tiger Woods during 13 of his 15 major victories, as well as helping Australia's Adam Scott to his 2013 Masters win at Augusta.
"We've definitely been a lot more disciplined about going to the range and putting green, chipping green after the round and making sure we're staying on top of it," Day said.
Marc Leishman is next best of the Australians having carded a 70 to remain at four-under, while countrymen Cameron Davis (68) and Matt Jones (69) are two strokes further behind.
With the 36-hole cut falling at two-under, Australians Curtis Luck (one-under) Rod Pampling (even-par), Aaron Baddeley (two-over) and Cameron Smith (three-over) all missed the weekend rounds.