With his second loss in his last four games, a Minnetonka grandmaster fell down in the standings as the U.S. Chess Championship tournament passed the halfway mark Tuesday.

Grandmaster Wesley So lost to his former college roommate, Grandmaster Ray Robson, after making what commentators called shockingly poor moves in time trouble.

Mathematically, So remains in contention with five rounds left to play. But realistically, he faces a big task in catching tournament leader Hikaru Nakamura, who remains undefeated. So now drops back from second place into a tie for third place, behind Robson.

Expert commentators were surprised by So's second defeat after his stellar play at recent high-stakes tournaments.

"The meltdown of Wesley, I'm still horrified," said Grandmaster Yasser Seirawan, providing livestream commentary at the tournament being held at the Chess Club and Scholastic Center of St. Louis.

For Robson, the victory was payback. Last October, he lost to So in the finals of the Millionaire Chess Open in Las Vegas, where So earned the $100,000 grand prize while Robson took home the $50,000 runner-up prize.

Dennis J. McGrath • 612-673-4293