Johnson leads at Northern Trust as Woods withdraws

JARED C. TILTON / GETTY IMAGES NORTH AMERICA / AFP

Dustin Johnson is on top of leaderboard at the halfway stage of the Northern Trust.

The American is one shot ahead of Jordan Spieth after two rounds of the PGA Tour's play-off event.

Meanwhile Tiger Woods was forced to withdrew before Friday's round because of "mild oblique strain that led to pain and stiffness".

The third round gets underway later on Saturday.

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