Ellen MacArthur says her bid to break the west-east solo trans-Atlantic speed record hangs in the balance.
The British sailor is travelling from New York bound to the Lizard Point off the south-west coast of England and is just two hours off schedule.
But she told BBC Sport: "The toughest part is coming up and it really could go either way at this stage.
"But I'm confident I can do it. I only have two hours to make up so it's going to be really tight."
MacArthur is chasing a time of seven days, two hours, 34 minutes, 42 seconds set by Laurent Bourgnon in 1994.
And she is expected to arrive in England in the early hours of Tuesday.
Her boat, B&Q, must cross the finish line by 00:44:42 GMT to set a new world mark.
She admitted the rigours of the challenge were taking their toll.
She said: "It's pretty tiring - the most undisturbed sleep I've had so far is 40 minutes. So it's beginning to get to me.
"But it's not affecting me too badly. I just have to hope the weather conditions stay on my side."