Second round leaderboard: -6 C Wi (SKo) M Kuchar (US) -5 M Leishman (Aus) -4 I Poulter (Eng) P Casey (Eng) L Donald (Eng) R Goosen (SA), D Johnson (US)
Selected others: -3 J Rose (Eng), H Mahan (US) E M Sim (Aus) +1 P Mickelson (US) +3 T Woods (US) +4 M Laird (Sco) +8 R McIlroy (NI)
Paul Casey was left out of Europe's Ryder Cup team
Europe Ryder Cup reject Paul Casey led a strong English showing in the second round of the BMW Championship.
Casey posted a second successive 69 in Chicago to finish four under par, two shots off joint leaders Charlie Wi of South Korea and American Matt Kuchar.
Englishmen Ian Poulter and Luke Donald are also four under, with compatriot Justin Rose a further shot behind.
Tiger Woods's hopes of qualifying for the Tour Championship are fading after a 73 saw him slip to three over par.
World number one Woods is likely to need a top-five finish to make the 30-strong field at the lucrative season-ending event, but is currently tied for 40th in a tournament with no halfway cut.
The 34-year-old American is the defending champion and a five-time winner at Cog Hill, where he had not shot back-to-back over-par rounds since 1994.
"Monty is a friend - he was in a very difficult position
Meanwhile, Casey made a mockery of Europe captain Colin Montgomerie's decision not to select him for October's Ryder Cup clash against the United States with another sublime two-under-par round, which featured four birdies and two bogeys.
Casey missed out on one of Montgomerie's three wildcard picks, with Donald, Padraig Harrington and Edoardo Molinari getting the nod.
"It's done and dusted," said Casey. "You've got to get over things pretty quickly - I got over it pretty quickly.
"Monty is a friend. He was in a very difficult position."
Rose, another player overlooked by Montgomerie, was also in the group at four under before a bogey at the 15th saw him drop back.
Poulter fell back one shot overall after a 72, while Donald carded a 70 to put himself in the group at four under, which also contains South Africa's Retief Goosen and Dustin Johnson of the US.
First-round leader Kuchar bogeyed four of his first six holes as he struggled with a virus, but played the last seven in three under to draw level with Wi.
Marc Leishman of Australia carded the best round of the day with a 65 to move into second place at five under.
It was another day to forget for Rory McIlroy of Northern Ireland, who shot a three-over-par 74 to finish eight over.