After two holes, Casey had the lead to himself at Muirfield Village in Dublin, Ohio. He went on to another six-under-par 66 to build a three-shot lead over
Casey was at 12-under 132.
McIlroy opened with a 63 and was 15 shots worse Friday. He didn't blame a sore left knee as much as his inability to hit fairways. He made three straight double bogeys, had a nine-hole score of 40 or worse in his fourth straight
Watson got within one shot of Casey late in the round until closing with back-to-back bogeys for a 69.
Jennifer Johnson matched the course record with a nine-under 62 on Friday to take the first-round lead in the ShopRite
The 22-year-old Johnson, the winner last year in Mobile, Alabama, had 10 birdies — five straight on Nos. 9-13 — and a bogey on the Bay Course at Stockton Seaview Hotel and Golf Club in Galloway Township, N.J.
The only other players to shoot 62 at Seaview were Laura Davies and Jimin Kang in 2005. Haru Nomura had a bogey-free 63, and Christina Kim opened with a 64.
Top-ranked Inbee Park and Na Yeon Choi shot 66 in the morning session. Park, winless this season, missed the cut last week in Alabama to end her streak at 22 events.
Second-ranked Stacy Lewis and Michelle Wie shot 67, and 17-year-old Lydia Ko was another stroke back.