Jason Day, the 2010 Nelson champion, withdrew Wednesday from the AT&T Byron Nelson Championship. No reason was given.
He is replaced by Tim Petrovic in the 156-player field.
Day, 27, is No. 8 in the World Golf rankings and 13th in the PGA Tour's FedEx Cup standings.
He won the Farmers Insurance Open in February and has made the cut in nine of 10 tournaments this season, with earnings of $2.4 million.
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Ian Poulter tweeted that he has also withdrawn due to a "slight strain" caused by working out.
Vaughn Taylor will replace Poulter.
Tournament play begins Thursday.
Eric Zarate, 817-390-7237