Five time champion Roger Federer faces the biggest test of US Open credentials to date when he takes on Gael Monfils for a place in the semi-finals.

The pair are just getting underway in the last men's quarterfinal, and while Monfils has only ever beaten Federer indoors, the Frenchman's strong serve and back court power have seen him shine in the first four rounds.

The winner will face Croatian Marin Cilic, who won through to his first US Open semi with a straight-sets win over Thomas Berdych.

Cilic is looking forward to watching Federer and Monfils, and take the chance to relax ahead of his semifinal.

Monfils has never reached the semifinals of a hard-court grand slam.