The world number one advanced into the quarter-finals at Flushing Meadows with a routine 6-3 6-3 win over Kaia Kanepi of Estonia, taking the American within three match wins of an 18th singles major.
Williams' form, by her own lofty standards, has been off in season 2014, and she said for the best part of eight months she was below par - despite collecting three titles between January and August.
"I'm playing a little better I think than I was in the beginning of the year. I'm also a little more relaxed," the top seed said.
"I think I played really well in Brisbane. I didn't play well again until I think maybe Cincinnati. So just being more consistent."
Williams did concede her own assessment of her season was down to her high expectations, as she reached the last eight of a major for the first time in 2014.
"Well, I'm a perfectionist, I always want to be the best, do the best," Williams said.
"I think I put a lot of pressure on myself to do well, particularly in the slams, to do really well. [I] had a couple nagging injuries that definitely didn't help. But other than that it was just that.
"Now I'm more relaxed. I feel like I don't have to win any more. I've had a wonderful career.
"Tennis has given me so much, things I would have never expected in my life.
"I feel honoured to even be a part of such a wonderful sport. So I think ultimately that really helps me to be able to be more relaxed."
Next up for Williams in New York is 11th seed Flavia Pennetta of Italy, who has made it to the quarters of the US major for a fifth time.
Despite downing Pennetta 6-2 6-2 on her way to the Cincinnati title in August, Williams said she will have a tough challenge on her hands come Wednesday.
"I don't think I have easy matches against her. I think she plays really well," Williams said of her fellow 32-year-old.
"I think the older she gets the better she plays, to be honest.
"It is not an easy match-up for me. We played in Cincinnati. I won. But she played well.
"This is a new week. She obviously learned a lot from that experience and she's going to come out ready to fire and to play again."