Roger Federer pulled off a great escape by fighting off two match points to beat Frenchman Gael Monfils and reach the semi-finals of the US Open last night, keeping his bid for an 18th grand slam on track.

The stirring 4-6 3-6 6-4 7-5 6-2 comeback marks the ninth time in his career Federer has rallied from two sets down to snatch victory, setting up a final-four meeting with 14th seed Marin Cilic.

Sixth seeded Tomas Berdych had no chance of conjuring a similar escape and lost 6-2 6-4 7-6(4) to big-serving Cilic, who is making his return to Flushing Meadows after missing last year's Grand Slam due to a doping suspension.

Federer, playing under the floodlights of Arthur Ashe Stadium court, must have wondered which Monfils would show up.

Labelled one of the game’s great natural talents, Monfils can be wildly entertaining or maddening, depending on his mood and the near capacity crowd saw both sides of the 28-year-old Frenchman during a riveting the three hour, 20 minute encounter.

Monfils added several highlight reel shots to his collection in winning the first two sets, and after Federer won the third he carved out two match points in the fourth.

But with Federer on the ropes the 20th seeded Frenchman could not deliver the knockout punch.

“When I was down two match points, I wasn’t feeling so great . . . I thought ‘this is it, this is the last point, man,” said Federer. “Just go down fighting, don’t miss an easy shot and let him have it.”

Misfiring most of the night, Federer slowly began to find his mark and seized his chance when it arrived, holding serve for 5-5 to close out the fourth set and then breaking Monfils in the first game of the fifth.

“It was one of those moments where you got the back against the wall and hope to get a bit lucky and you hope to play exactly the right shots that you need or that he completely just messes it up,” said the 33-year-old.

“Either way works as long as you get out of it. It’s just unbelievable to win matches like this at slams.

“I’m not sure I have ever saved match point before in a slam. If that hasn’t happened, I’m unbelievably happy that it was today.”

Cilic, who pounded out 19 aces, had Berdych under pressure right from the start, breaking the Czech at the first opportunity in the first two sets to grab a 2-0 lead.

Berdych turned the tables on Cilic in the third and raced out to a 3-0 lead but the Croatian, relying on his booming serve, broke back and forced a tiebreak before clinching the match with a thundering ace and forehand winner on the final two points.

It has been a turbulent few years since Cilic’s last grand slam semi-final at the 2010 Australian Open.

His problems came to a head last year when he was handed a nine months drug suspension for taking a banned stimulant which was later reduced to four months but he was still unable to compete at the US Open.

"I mean, it was a difficult period. I didn't know when I'm going to start back," said Cilic, who becomes the first Croatian to reach the semi-finals since his coach Goran Ivanisevic in 1996.

“But it was also a good period for me. I matured a bit more and I was working day after day. “Of course it was a huge motivation for me when I came back.”

Certainly since his return last October the 14th seed has been firing on all cylinders, winning events in Zagreb and Delray Beach while reaching the final in the Rotterdam.

His 45 match wins this season are second only to Federer (54) and his run to the Flushing Meadows semis has included wins over 26th seed Gilles Simon and 18th seed Kevin Anderson.

“Even though I had great success beginning of the year, I felt that I started to play really well somewhere from French Open,” said Cilic. “Since then I think the things are in good place for me and moving really, really good with everything.”

“When I came back to the tennis court I erased it from my memory. I just used the positive parts.”