Ellen MacArthur saw her charge towards the solo non-stop round-the-world record become more of a struggle as she waited for stronger winds on Sunday.
The 28-year-old was almost a day ahead of Frenchman Francis Joyon's mark during Saturday night, but that lead has slipped to just under 18 hours.
MacArthur told her website: "For seven hours we have been sailing under-powered, waiting for the cold front.
"I dare not put up more sail because I know the wind is going to pump soon."
MacArthur expects to face winds of 30-40 knots in the coming day, with waves peaking at 30ft.
However, there is a threat conditions could become much worse by the end of next week.
She said: "I'm really worried about the big depression forming in the future on our track that I don't think we will be able to avoid.
"At the moment I don't know how we are going to avoid it. We have to avoid the 50-to-60 knots at all costs, even if we actually stop."
MacArthur has now sailed 8,505 miles of the 26,000-mile course and has already smashed Joyon's record from the English Channel to the Cape of
Good Hope in Africa by 10 hours 45 minutes.