The Croatian 14th seed was too good for Czech sixth seed Tomas Berdych in a 6-2 6-4 7-6 (7-4) win at Flushing Meadows in their quarter-final.
Cilic's last-four appearance marks the second time he has been in a grand slam semi-final – and first since the 2010 Australian Open.
The 25-year-old said he was at his best in New York, but refused to be satisfied too easily.
"I feel I'm playing probably the best tennis of my career. I think what helped me also was the consistent play over last five, six months," Cilic said.
"This year I haven't lost too many matches against guys who are lower ranked than me. That's always very positive.
"So overall in the game I feel I'm playing well but I'm not getting satisfied too easy. I'm happy, but I still want to do well.
"I feel that it's still a lot to improve even in some matches. I want to keep going.
"Now I made it to the semis and it's completely different for me now atmosphere than what I have been in 2010.
"At that time was maybe a little bit, too shocked for me and I didn't know how to deal with it afterwards. But now I'm, you know, cool and it's going well."
Cilic said he deserved his semi-final appearance in Melbourne in 2010, but felt he could still improve on his current form.
"I would say I deserved it at that time, too, to be in [a] semi-final, but I was putting more pressure on myself with the expectations," he said.
"I wanted to do too well. Like on each match, on each training session, I was looking from my own self for too much.
"Then after a couple of months everything breaks, so I feel now I'm steadily progressed a lot. I feel like I'm playing well but still I want to, I feel like I can still improve."