Day and Jones recorded rounds of 5 under 67 in ideal conditions, both having played the North Course on day one although the more demanding South Course awaits them on day two.
The pair is five shots behind the 2017 champion Jon Rahm whose round of 62 leaves him one ahead of Justin Rose and tournament invite Doug Ghim.
Chinese Taipei’s C.T Pan is alone in 4th place at 8 under and Jordan Spieth 5th after an improved round of 65.
Day is a two time winner of this event including when defeating Alex Noren and Ryan Palmer in a playoff twelve months ago and in 2015 when also safely negotiating a playoff.
Day was also runner-up in 2016 and won a World Junior title in his amateur days at the venue.
Of the other Australians in the field John Senden and Marc Leishman have begun well with rounds of 68, Leishman twice a runner-up at Torrey Pines. Both Leishman and Senden, too, played the North Course on day one.
Adam Scott, a proponent of the flag in method of putting, was round in 70 on the South Course, Cameron Smith had 71, Cameron Davis 72 and Rod Pampling 75.
Scott is taking the new ruling on being able to leave the flag into his tournament routine even on putts as short as 12 feet as shown in the video below and even shorter on some occasions.
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