The 31-year-old, who is the first Chinese player to reach the last four at Flushing Meadows, was humbled 6-0 6-2 in 87 minutes on Arthur Ashe.
"It's not about the technique, it's about the problem with myself," Reuters quotes the Chinese player as saying.
"First time in the semis, I was thinking about the situation. Today, it doesn't matter who the opponent is. The problem is myself. Even if today I was playing against [a player with] the ranking of 100, [there would] still [be] the same problem.
"When I walked to the court, I was feeling like the court was so big. My side was feeling like a football court. I cannot focus. In the end, finally I can play tennis."
Williams will play Victoria Azarenka in the final.