Luke Donald and Rory McIlroy will go head-to-head in the first two rounds of the Memorial tournament in Ohio on Thursday and Friday.
McIlroy lost top spot again to his Ryder Cup team-mate when he crashed out of the BMW PGA Championship at Wentworth last week and Donald went on to retain the title.
They could swap places again, but 23-year-old Northern Irishman McIlroy has to win and even then it will not be good enough if Donald finishes in the top 13.
Tiger Woods is also in the field, as are Masters champion Bubba Watson following a month-long lay-off and Dustin Johnson, who has not played since March because of a back injury.
Paul Casey was due to have played, but has decided to give his injured shoulder another rest after struggling at Wentworth.
Casey suffered a dislocation snowboarding on Christmas Eve, missed the first two months of the season and then suffered a setback at the Players Championship three weeks ago.
The former world number three played only nine holes there, withdrew from the Volvo World Match Play in Spain and last week had rounds of 78 and 76 to miss the halfway cut by nine shots - one more than McIlroy.
