The 14-time Grand Slam champion, a three-time US Open winner coming off a Wimbledon title in July and London Olympic gold last month, meets Italian 10th seed Sara Errani on Friday to decide a berth in Saturday's US Open final.
"I feel like I'm going to get more focused and serious and start playing 'Serena tennis' in the next couple of rounds," Williams said.
"I didn't think I came into this tournament playing my best but I definitely played better in the last two matches. I hope I have two matches left and give 200 percent."
Williams is going for a rare treble by adding the US Open to her recent trophy haul. Only her sister Venus Williams in 2000 and Steffi Graff in 1988 have taken Wimbledon and US Open titles and Olympic singles gold the same year.
Asked if she felt close to realizing her potential as a player, Williams replied, "I'm closer than I have been in the past. I'm never satisfied but I'm coming closer."
Williams has fired a tournament-best 41 aces, including 12 in Wednesday's 6-1, 6-3 victory over outmatched Serbian 12th seed Ana Ivanovic, a former World No. 1 who won a Grand Slam title at the 2008 French Open.
"I served more consistently. My serve hasn't been the greatest for me at this tournament so I was glad about that," she said. "Some days my serve isn't as consistent or I'm not hitting it as well as I would like."
Williams put together a run of 23 consecutive games won until Ivanovic stopped it in the first set, but said she wasn't sure if that was a big deal.
"I don't know? I don't keep up with those stats," she said. "I felt much better moving. I was excited I'm moving faster. Feels good. I felt solid.
"I really just try to focus on my game pretty much. Just trying to do the best that I can."
Ivanovic said there was little else she could do to try and deny Williams a seventh US Open semi-final appearance.
"Her serves were too good. She was serving unbelievable," Ivanovic said.
"She was hardly missing. That puts a lot of pressure on opponents' own service games. Sometimes I was rushing too much and wanting too much to hold onto my service games because I felt I was nowhere near the chances on her serves."
"She has always had a great serve. I think it's natural. I wish I had that kind of serve."
Williams has hammered in 58 percent of her first serves, 45 percent of them for winners without making another swing, and surrendered only 12 double faults.
"She's definitely making more first serves," Ivanovic said. "The power, it's consistent. She was consistently serving 116, 118 mph. It's hard and the placement is so good. It's hard to read it."
Williams credits some of her success to extra work under her father Richard in her youth and began a heavy serve practice regimen before winning at Wimbledon.
"My dad always had us hit a lot of serves," Williams said. "Before I played Wimbledon I hit a lot of serves, tons and tons and tons of serves. Definitely depends on how I'm feeling or how I feel about certain shots."