Westwood played a key role in Europe's Ryder Cup triumph
Ryder Cup star Lee Westwood suffered a recurrence of a calf injury on day one of the Dunhill Links Championship and may withdraw from the event.
Westwood, who carded a two-under-par 70 at Kingsbarns to lie four off the lead, would usurp Tiger Woods as world number one with a top-two finish in the event.
Scot Martin Laird is joint leader after carding a 66, also at Kingsbarns.
"Playing two weeks in a row was probably too much in hindsight," said Englishman Westwood.
"I felt it coming off the seventh tee. It was a really steep downslope and it's really aching now like I'm doing too much too soon.
"I should have gone back to rehab. I'm going to go and get treatment and see how it is, but I don't want to get back to where it was.
"It improved a lot, but it's still not right. I can't load my right side. I just want to get off it as quickly as possible."
The Ryder Cup, in which Europe were victorious over the United States with Westwood scoring two-and-a-half points out of a possible four, was the 37-year-old's first competitive golf since August.
Laird, who plays most of his golf in the United States, shares the lead with Dutchman Maarten Lafeber (Carnoustie) and Argentine Ricardo Gonzalez (St Andrews).
Martin Kaymer, who carded a 68 at Kingsbarns, is the highest-placed Ryder Cup player, while England's Ross Fisher carded a 69, also at Kingsbarns. Nine of Europe's Ryder Cup players are in the field, plus captain Colin Montgomerie.
The Dunhill Links Championship features 168 professionals and the same number of amateurs playing in two man teams.
The tournament is played on rotation on three courses, Carnoustie, Kingsbarns and St Andrews, with the latter also hosting Sunday's final round.