A day after Minnetonka Grandmaster Wesley So had rebounded and seemingly pulled himself back into contention at the U.S. Chess Championship, he suffered his most surprising loss yet.

With his third defeat in eight games Thursday, So has put himself into a deep hole in terms of his hopes of winning the national title.

The latest loss was to the lowest-rated player in the tournament, Grandmaster Conrad Holt, who had managed only one win in the first seven rounds, before his win over So.

So, the No. 8 player in the world, is known for his steady, reliable play — recently going more than 50 games without a loss — so for him to lose three games in short order is extraordinary. On Wednesday, when he was answering questions about his two loses up to that point, So described the tournament as one of his worst.

In Thursday’s game, So had the black pieces and he had to contend with a surprise by Holt on the 12th move. That’s a point in the game when elite players usually are still “in book,” meaning they are following opening variations that have been time-tested and committed to their expansive memory. In this case, Holt deviated from the standard line, turning to a move that he had explored and prepared months ago. It was such an unusual move that in a database of more than 6 million games, most of them among top players, this position had never been reached, commentator and Grandmaster Maurice Ashley explained in a livestream broadcast.

Initially, So navigated correctly, according to the analysis by powerful chess engines. In fact, he made one move that was counterintuitive, but which the engines endorsed. But So’s 20th move failed to put the kind of pressure on Holt that he no doubt envisioned, and then there was no recovery. So resigned after Holt’s 35th move.

As So lost, his two chief rivals pulled farther ahead, with Grandmaster Hikaru Nakamura winning and taking over the sole lead with 6 points, and Grandmaster Ray Robson drawing — falling a half point behind Nakamura, but stretching his lead over So to a full point. Players get one point for a win, a half point for a draw and nothing for a loss.

Play continues Friday, Saturday and Sunday at the Chess Club and Scholastic Center of St. Louis, with the champion crowned Sunday — or in a Monday playoff if there’s a tie after the final round on Sunday.