The world number one was knocked out in the third round at SW19 earlier this year, but secured her first Grand Slam final of the year with a straight-sets win over Russia's Ekaterina Makarova last night.
The American told BBC Sport: "There's always sceptics. I worked really hard for Wimbledon, and I was really disappointed and sad and shocked that I wasn't able to win.
"I worked hours, more than I worked before. Maybe it's just paying off now."
Williams will be looking for her third consecutive title at Flushing Meadows when she faces Denmark's Caroline Wozniacki in the final tomorrow.