Mr Guo was greatly admired in China for his love of his country and his family.
He dedicated his record-breaking achievement to his late father, and in one of his most recent reports from his voyage he spoke of how he listened to recordings of his young son laughing. He called it “the most beautiful song in the world”.
Zhai Feng, a close friend of Mr Guo and a fellow sailor, said: “I have been in shock since I heard this news.
“We know that falling into the water is certain death. If you cannot climb onto the boat within half-an-hour, you will surely be dead.
“My wife cried floods of tears when we learnt the news, and we are both praying for him.”
What happened to Guo remained unclear.
Guo’s team said they had observed his speed slowed on Tuesday and attempted to contact him, but he did not answer either satellite calls or internet communication.
The US searchers had found a broken sail in the water, they added.
The US Coast Guard said it was called when Guo’s team had not received notification from him for 24 hours.
The sailor had previously been “in constant contact” with his shore team and family and was “not likely to miss scheduled calls”, it added.
Chinese fans expressed fears for the mariner, with one writing that it was “likely he was adjusting or repairing the sail when he was struck or an accident occurred and he fell”.
Others urged rescuers to continue the search.
“Absolutely do not stop the search and rescue! It’s only 24 hours, the water is warm enough, Guo Chuan’s physical abilities are fine, if money’s an issue the shore team must immediately open a donation account… You can’t give up!”
But a tone of mourning and tribute also appeared, with one writing that sailors, like mountain climbers, embraced challenge to access a vast perspective, with “captains’ hearts always facing the sea”.