Should Spieth go on and win this week he would move inside the top ten in the world ranking for the first time from his current 17th and further confirm that he is on the way to the absolute elite of the game.
Cink has not won since the 2009 Open Championship where he defeated Tom Watson in a playoff while the long hitting Colsaerts now appears entrenched on the PGA Tour.
Leishman followed up his very good 5th place finish two weeks ago at the Sony Open to lead the Australians. His second round of 71 at the South Course was blotted by a double bogey early in his back nine but he looks well placed to challenge his previous best at Torrey Pines when runner-up in 2010.
Jason Day faltered a little with a second round of 73 over the South Course in the company of Phil Mickelson and Bubba Watson to be in a share of 11th position and five shots from the lead.
Matt Jones added a round of 65 to be in a share of 20th, Stuart Appleby is in 50th position, with Aaron Baddeley and Steve Bowditch 66th.