(REUTERS) - Danny Willett, already in mediocre form ahead of his Masters defence, suffered a potential setback when he pulled out of the Arnold Palmer Invitational, citing illness on Thursday.

Willett withdrew shortly before his scheduled tee time, allowing an alternate to take his place in the field at Bay Hill in Orlando, Florida.

"Really sad to withdraw," the Englishman said on Twitter. "I've been up all night with terrible sickness. Not ideal but felt it worse ... to take spot from reserve if I wasn't able to finish."

Willett still has three weeks to hone his game for the year's first Major championship.

He has played only five tournaments worldwide this year, with one finish inside the top 50 - a tie for fifth at the Maybank Championship in Malaysia.