Rose was in superb form as he edged into the lead
US unless stated
-6 J Rose (Eng), R Allenby (Aus)
-4 T Woods, A Baddeley, O Browne
-3 L Mattiace, R Karlsson (Swe), JJ Henry, S Micheel
-1 V Singh (Fij)
+4 G Owen (Eng), B Davis (Eng)
Justin Rose shot a two-under round of 69 to take over from Tiger Woods at the top of the leaderboard at the Deutsche Bank championship in Boston.
Rose shares the lead with Australian Robert Allenby, who hit a purple patch in his round of 66.
Starting on the 10th, Allenby birdied the 15th before a hole in one on the 16th, followed by birdies on 17 and 18.
Woods had three bogies on the back nine but he recovered to record a one-over 72 and reach halfway on four under.
The Open and USPGA champion, who is aiming for his fifth straight victory, struggled from the tee throughout - missing seven fairways.
My short game kept me in there
Woods is with Aaron Baddeley and defending champion Olin Browne.
Rose had a good start with two birdies in his first three holes but dropped shots at the 7th and 9th.
But the 26-year-old Briton, who is currently 103rd on the US money list, recovered with two more birdies at the 11th and 16th.
"It was a battle and grinding sort of day," he said.
"The course is playing very, very difficult. There was a lot of cross winds today so it was difficult to get the ball close to the flag.
"But it didn't matter for me because when I managed to miss the green I had a good up-and-down or managed to chip in.
"That was the key for me today. My short game kept me in there."
Sweden's Robert Karlsson is three off the lead after a level par 71 but Carl Pettersson failed in his late bid to impress European Ryder Cup skipper Ian Woosnam.
Pettersson, who would have been leading the points race if he had been a member of the European tour from the start of qualifying, added a 75 to his opening 75 to miss the cut by seven shots.
English duo Greg Owen and Brian Davis missed the cut by one shot after finishing the event on four over.
Owen carded a second round 73 while Davis had six bogeys and a double bogey in a disappointing 77.
Fiji's Vijay Singh, the winner of this event in 2004, shot a 71 and is one-under for the tournament.