A superb six-under-par 65 has given Swede Carl Pettersson the chance of a second US Tour victory in six weeks and boosted his hopes of a Ryder Cup debut.
But Pettersson, winner of the Memorial Tournament at the start of last month, will have to overtake world number two Vijay Singh and hold off world number one Tiger Woods to take the Cialis Western Open in Illinois.
Only 35th at halfway, Pettersson will go into the final round in joint second place, two behind Singh, while Woods is one further back in a tie for seventh spot following a 66.
Pettersson is eligible for September's Ryder Cup match after rejoining the European tour this season. Lifting the title tomorrow could put him fifth in the cup standings.
Alongside him on nine under are Americans Stewart Cink and Joe Ogilvie, Australian Matthew Goggin and South African Trevor Immelman.
Pettersson's compatriot Daniel Chopra, who was two clear of Singh at halfway, fell away to 21st with a 76.
Woods was in danger of a second successive missed cut following his opening one-over 72, but has looked much more like his old self since.
Phil Mickelson, though, clearly has work to do on his game. He began the week well enough with a 67 to be just one off the lead, but scores of 74 and 75 sent the Masters champion tumbling to 75th spot.
Justin Rose, who made it through to the weekend right on the limit of level par, fired a 65 and is in a tie for 15th on six under, while fellow Englishman Luke Donald was just one further back alongside Chopra.
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