Welsh skipper Alex Thomson slipped to third in the Vendee Globe overnight after his boat Hugo Boss suffered a "possible structural issue on board", race organisers announced on Sunday.
Thomson, who is reported to be safe, was lying second behind Thomas Ruyant, heading south-east around 800 miles east of Rio de Janeiro, when his team alerted the organisers to the problem.
Thomson, who led the solo round-the-world race by 90 nautical miles earlier in the week, cut his speed from around 16 knots to six.
"Thomson and his team, together with their appointed naval architects and structural engineers, are now working together to assess the extent of the structural issue and to determine a repair programme and timeline," the Welshman's team said in their statement.
"Thomson is safe and well onboard, and in regular dialogue with the team."
Thomson lost considerable ground on Ruyant's LinkedOut and gave up second place when Charlie Dalin passed him in his boat Apivia just after midnight GMT)
"For Alex I hope it's not too bad and the extent of the damage is limited," said Dalin.
"Above all, I hope that this does not mean the end of the Vendee Globe for him. We have a great race with him, and this trio we are in is very stimulating."
French skipper Ruyant leads the race which started two weeks ago in Brittany. He is 38.5 nautical miles ahead of compatriot Dalin and 73.1nm in front of Thomson.
The breakaway trio, who are all in the new generation of 'foilers' -- boats equipped with foils which help lift the boat so that it is virtually flying across the top of the waves -- are racing in light, variable winds.
"I have the impression that I am reliving a doldrums passage but it is even more bizarre," said Dalin.
"I found some pretty strange variable winds, sudden changes in direction and force, I really didn't expect that. I even got gusts from the north-west yesterday.
"This morning (Sunday), the sea is flatter and there is a beautiful starry sky, it is beautiful."
So far in the competition, there has only been one abandonment, that of Nicolas Troussel after a dismasting on Monday off Cape Verde.
Jeremie Beyou, also in a 'foiler' had to return to the start to repair his boat Charal before setting off again on Tuesday.
The finishers will complete approximately 24,296 nautical miles around the globe before they finish back in France at some point in January.