As the US Championship (www.USChessChamps.com) enters its second half in St. Louis, we find Brandeis grad and Bay Area native Sam Shankland in clear first with an undefeated 4.5/6. Close behind are Wesley So and our pre-tournament favorite, Fabiano Caruana, both with 4/6. Shankland has shown good form and should have good chances ahead if his nerves stay strong.

For Caruana, it has been a strange event. In the first round, he faced the youngest and lowest-rated player in the field, Awonder Liang, 14. Unexpectedly, it was a comfortable draw for Liang. Then in the fourth, Caruana lost to the low-rated Zviad Izoria. He played a dubious novelty against Shankland in the fifth and had to fight to hold the draw a pawn down for most of the game. But then he beat Ray Robson in the sixth to get to 4/6 and second place. Even so, he is very much in the running.

So has played steady if boring chess — four draws, two wins. Another pre-tournament favorite, Hikaru Nakamura, finds himself in a very surprising situation, a two-way tie for fifth, having managed to just draw all his games. This has to be disappointing for a world top 10 player, though one gets the feeling that he’s the midst of a transition from chess into financial trading. At the bottom of the table we find Robson with 2/6, in a tie for last place with Alex Onischuk. Robson’s poor play here and his lack of activity of late might indicate yet another player in transition.

The Women’s Championship finds an unusual situation, with 15-year-old Annie Wang of California in clear first with an undefeated 5/6. She still has to face the two highest-rated players and multiple-time champions Irina Krush (7) and Anna Zatonskih (4), and it will be very interesting how she handles this pressure. A champion two years ago, Nazi Paikidze of Las Vegas, is a half-point behind Wang, and well-positioned for a run. Last year’s champion, Sabina-Francesca Foisor, is mired in a tie for eighth with 2.5/6

Coming events: May 6, MIT Open, MIT Stata Center, 32 Vassar St., Cambridge,May 26-29; Mass Open (State Championship), Best Western Royal Plaza Hotel, 181 Boston Post Road West, Marlborough

Recent results: MCC April Fool’s Swiss Open, 1st-2nd, Matthew Fishbein, John Chamberlin, 3/4; U2000, 1st-2nd: Ed Astrachan, Ethan Thompson, 3.5/4

Answer to today’s problem: 1.Qxg6+! Kxg6 2.Ne7+! Kh7 3.Bd3+ Rf5 4.Bxf5#

Chris Chase can be reached at BostonGlobeChessNotes@gmail.com.