It was another low in a miserable year for the eighth-seeded Dane, who also lost in the first round of Wimbledon, but there were extenuating circumstances as she had her right knee heavily taped.
Wozniacki sustained the injury bidding for her fifth straight New Haven title last week and pulled out during her semi-final against Maria Kirilenko.
The 22-year-old said: "You always want to go in and do your best no matter what's happening out there. I tried.
"I didn't play particularly well, made too many errors. It's unfortunate because it's a huge tournament, a tournament you want to play well in.
"I think it's a temporary issue. Hopefully it will get better quickly. It's frustrating to have some injuries, but it happens to everyone. It's just about moving on."
Wozniacki refused to be too downhearted about her season, adding: "The year's not done yet. Definitely the grand slams this year haven't been great.
"After the year's finished you can evaluate, you can see what was good and wasn't so good and work from there. I still have plenty of years in me. Hopefully I can just turn it around and play even better."