Marin Cilic reacts after beating Roger Federer.

Last year Marin Cilic watched the US Open from home, serving a four-month doping ban for using a banned stimulant he said he had unknowingly taken.

On Saturday, the big Croat played the best tennis of his life to upset 17-times grand slam singles winner Roger Federer and reach the finals of the US Open.

“From first point to the last I was absolutely playing the best tennis of my life,” the bearded, 14th-seeded Cilic said after his 6-3 6-4 6-4 one hour 45 minute rout of the second-seeded Swiss.

“Considering the huge occasion I was playing in, for the second time in a semi-finals of a grand slam, it just can’t be more special.”

Cilic used his time banished from the court to dedicate himself more deeply to the game. He worked to improve his strength and conditioning and hired his idol Goran Ivanisevic to help him harness the power of his serve.

“I was at home and was working, preparing as much as I could and was using every day to train,” said the 6-foot-6 (1.98 m) Cilic.

Hard work paid off, and Cilic and fellow giant-killer Kei Nishikori of Japan, who eliminated world number one Novak Djokovic in the other semi-final, will meet in a most unlikely men’s final tonight.

Cilic said he took inspiration from Swiss Stan Wawrinka, who broke through for his first grand slam title by beating Rafa Nadal this year in the Australian Open final.

“Just being able to see that he’s able to beat those guys,” Cilic said gave him a lift.

“OK, he was close. Last year semis here. But sort of he made that huge jump in short period of time.”

Cilic had lost all five previous matches against Federer, but the improving Croat gave the Swiss master all he could handle in a 7-6 6-7 6-4 loss in their most recent meeting in Toronto last month.

“Over there in Canada, confidence-wise and belief-wise, showed me that if I play well I can have a chance. Over there Roger was causing me much more trouble on his serve and I wasn’t getting as many returns in the court like today,” Cilic said.

“I felt today the return was extremely good, especially on the second serve and that opened up many more opportunities for me.”

Cilic took advantage by blasting 43 winners, 15 more than Federer, and his confident returning game allowed the second seed to win just 71 per cent of his first serve points and only 48 per cent of his second serve points.

“I felt I was hitting the ball extremely clean. I have some days where I am missing a lot of balls, but today it was very pure from my end of the court.

“To be in this position, I was working for all my life,” added the 25-year-old Croat.

“When you are young on the tour you always feel you have enough time. You have a lot of grand slams.

“Just to be in this situation, I can say this moment is extremely huge achievement.”