Englishman Luke Donald could drop to second in the world rankings after a sluggish start to the RBC Heritage event in South Carolina.
The 34-year-old, who must finish in the top eight at Hilton Head to remain number one, is eight shots off the lead of Chad Campbell, Vaughn Taylor and Colt Knost after a four-over opening round of 75.
Donald spent 40 weeks at golf's summit after a consistent 2011 before being deposed by US Open champion Rory McIlroy in March.
However, Donald regained top spot after triumphing at the Transitions Championship in Florida two weeks later.
A finish outside the top eight this weekend would see Northern Irishman McIlroy again move ahead of Donald in the world standings.
Luke Donald could lose number one spot this weekend
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