By Piers Newbery
BBC Sport at Flushing Meadows
|Venue: Flushing Meadows, New York Dates: 28 Aug-10 Sept|
|BBC coverage: Live radio and text commentary on selected matches every day.|
Roger Federer was taken to five sets for the second time in three days before overcoming Russia's Mikhail Youzhny at the US Open.
The five-time champion was two sets to one down but eventually came through 6-1 6-7 (3-7) 4-6 6-4 6-2 in New York.
Federer has now won all 17 matches against Youzhny, a year younger at 35, since the pair first met in 2000.
The Swiss third seed goes on to face Spanish 31st seed Feliciano Lopez in round three at Flushing Meadows.
He remains unbeaten in Grand Slam play this year, having won his 18th and 19th major titles at the Australian Open and Wimbledon, and skipped the French Open.
However, Federer's preparation for Flushing Meadows was disrupted by a back injury and he has been taken to the limit in his opening two matches.
American teenager Frances Tiafoe forced a fifth set in Tuesday's night session, before Youzhny led by two sets to one on Thursday afternoon.
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"These five-set battles are actually quite a lot of fun and I feel quite warmed up by now," a smiling Federer said after winning in three hours and eight minutes.
After breezing into a 4-0 lead, the errors began to flow from Federer's racquet.
His backhand, such a strength for much of the year, was particularly wayward and he finished with 35 errors off that side in a total of 68.
It was a poor forehand that allowed Youzhny back into the match as the Swiss served for a two-set lead, and it was a battle to the finish line from then on.
The Russian levelled in the tie-break and drew gasps from the 23,000 spectators as a forehand brought him the decisive break in the third.
A woeful Federer volley into the tramlines seemed to sum up his afternoon, but the 19-time Grand Slam champion gathered himself sufficiently to edge through.
After an early break in the fourth set he again failed to serve out, but this time responded in the next game to level when a Youzhny backhand flew wide.
The Russian was having his own problems by now, requiring visits from the doctor in the fourth and fifth sets and falling to the court, apparently with cramp, early in the decider.
A tired double-fault saw Youzhny fall behind at 4-2 and Federer quickly saw the match out against an opponent now unable to move freely, finishing with a smash.
"I haven't played a lot of guys with cramps in the last decade or so and the rules have changed, you're not allowed to get any help from the physio," added Federer.
"We've played a bunch of times and this was probably our best match."