Williams had not lost more than three games in a set all tournament in New York heading into her last-eight clash with Italian 11th seed Flavia Pennetta.
That statistic was on life support when Pennetta took the first three games, but Williams pulled through for a 6-3 6-2 victory on Arthur Ashe Stadium.
Williams came into the major with a 38-6 win-loss record on the WTA Tour in 2014, and the American said it was her competitive edge that got her back on track after a slow start.
"It definitely helped [being match hardened]," Williams said.
"I was thinking that, you know, I'm down two breaks? But I felt like it's not the end of the world. You can just win one game at a time. That's all I tried to do.
"I definitely think match play helps me out a lot getting into that mentality, getting into the groove, and getting back just getting back in things.
"I think it was really able to help me out, and I think at the end of the day was able to come back from that and get going."
Williams' semi-final opponent will be Russian 17th seed Ekaterina Makarova, who is into the last four at a major for the first time.
Makarova, a left-hander, famously beat Williams at the 2012 Australian Open, but the 17-time major winner has won their two contests since.
Williams said she was quite comfortable preparing for Makarova.
"I have played two lefties already and I often practice with Varvara [Lepchenko]. She's a lefty," Williams said.
"I think that's been good for me, so I'm ready for the lefty. I think it will be a good match.
"She's beaten me before. I am obviously going to go in there and just try to do the best I can."