Roger Federer was forced to come through another tense five-setter at the US Open on Thursday as he scraped to an unconvincing victory over Mikhail Youzhny.

The 36-year-old Swiss, who is seeking his third grand slam title of a remarkable year, was taken to a decider by American youngster Frances Tiafoe in his first-round match but initially made serene progress against Youzhny - a player he had beaten in each of their 16 previous meetings.

However, after losing the first set 6-1, Youzhny sensationally won the next two, aided by a string of mistakes from his illustrious opponent, whose fitness was in doubt coming into the tournament.

It was Youzhny who endured physical problems as the contest continued, the Russian troubled by apparent cramp as his hopes of a shock win ebbed away.

Federer eventually completed a 6-1 6-7 (3-7) 4-6 6-4 6-2 win in three hours and seven minutes to maintain his record of having never lost prior to the third round in New York, but he will be well aware significant improvement is needed if he is to win this event for a sixth time. 

There was no hint of the drama to follow as Federer breezed through the first five games in a supremely confident start. He was only denied a bagel when Youzhny saved two set points before eventually holding in a lengthy sixth game.

Set two was not so straightforward for the third seed, who initially fell 2-0 down and then, after reclaiming control, failed to serve out the set at 5-4.

Youzhny duly pulled level through a tie-break highlighted by two spectacular backhand winners from the Russian.

And as Federer's unforced error count continued to rise alarmingly, Youzhny was able to break for a 3-2 lead in the third and move within one set of a huge upset.

A turning point arrived when Federer broke from 40-15 down to claim a 3-1 lead in set four, with Youzhny calling for the trainer at the next changeover.

Although the underdog courageously got back on serve, his movement appeared increasingly compromised and Federer swiftly broke again to force a decider that he won with a degree of comfort despite failing to find his best form.

STATISTICAL BREAKDOWN
Federer [3] bt Youzhny 6-1 6-7 [3-7] 4-6 6-4 6-2

WINNERS/UNFORCED ERRORS
Federer - 63/68
Youzhny - 27/55

ACES/DOUBLE FAULTS
Federer - 12/2
Youzhny - 5/8

BREAK POINTS WON
Federer - 8/16
Youzhny - 4/8

FIRST SERVE PERCENTAGE
Federer - 62
Youzhny - 62

PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Federer - 78/51
Youzhny - 62/44

TOTAL POINTS
Federer - 174
Youzhny - 143