Roger Federer, Rafael Nadal at US Open Crossroads

Roger Federer and Rafael Nadal, with 31 major titles between them, head for the US Open with their Grand Slam careers at crossroads.

Despite turning 34, Federer was an impressive winner of a seventh Cincinnati Masters title last week, seeing off world number one Novak Djokovic in the final.

It was the Swiss star's 87th tour title and revenge for losing a second successive Wimbledon final to the Serb last month.

But Federer has stumbled badly in New York in recent years.

A five-time champion from 2004-2008, he was runner-up to Juan Martin del Potro in 2009 but hasn't been back to the final since.

His record at the Grand Slams in 2015 has also been mixed.

The seven-time Wimbledon champion may have at least reached the final at the All England Club, but that run was preceded by a shock third round exit to Italian journeyman Andreas Seppi at the Australian Open, his earliest in Melbourne in 14 years.

He then endured a quarter-final loss at Roland Garros to compatriot Stan Wawrinka.

Federer's last Grand Slam title came at Wimbledon in 2012 and if he wins the US Open this year, he would be the oldest champion since 35-year-old Ken Rosewall back in 1970.

"I have had to adjust my game accordingly," said Federer as he explained his longevity.

Federer faces Argentina's Leonardo Mayer, the world 33 in the first round.

In contrast to the Swiss player's staying power is Nadal's swift, brutal decline.

The 29-year-old Spaniard, the 2010 and 2013 champion in New York, has endured a tough year, the knee joints and wrists looking stiffer and weaker.

After being dethroned as French Open champion -- his quarter-final loss to Djokovic was only his second ever defeat in Paris -- Nadal slumped to 10 in the world, his lowest ranking for a decade.

- Murray v Kyrgios -

Nadal has lost 14 times already this year compared to 11 in the whole of 2014, seven in 2013 and just six in 2012.

He sat out the 2012 and 2014 US Opens because of injury.

At Wimbledon, he was knocked out in the second round by Germany's Dustin Brown while his North American hardcourt season ended in a quarter-final loss to Kei Nishikori in Montreal and a third round exit against Feliciano Lopez in Cincinnati.

"After a tough year, I feel close to being at the level of tennis where I have been in the past," said Nadal, who starts his campaign against 18-year-old Borna Coric of Croatia, who defeated the Spaniard in Basel last October.

Djokovic already has the Australian Open and Wimbledon titles under his belt in 2015, taking his majors total to nine.

The US Open champion in 2011, Djokovic was runner-up in 2012 and 2013 before suffering a shock semi-final loss to Nishikori 12 months ago.

The 28-year-old boasts a 56-5 record this year but two of those defeats have come this month -- to Andy Murray in the Montreal Masters final and then against Federer in Cincinnati.

Djokovic begins on Monday against Brazilian world number 89 Joao Souza, who has never won a main draw match at a Grand Slam.

"I set myself up for high standards because of the results I've had the last couple of years. I'm an ambitious guy and I devote myself completely to this sport," said Djokovic.

Murray, the world number three, won in New York in 2012, ending Britain's 76-year wait for a Grand Slam men's singles champion.

He has made at least the quarter-finals in New York in the last four years.

The 28-year-old has a mouth-watering opener against controversial Australian Nick Kyrgios, who is playing under the threat of an ATP suspension for his infamous sex slur against Stan Wawrinka in Montreal.

"He's a young guy. He made a mistake. All the guys here when they were 19 or 20 made mistakes," said Murray, who holds a 3-0 career lead over Kyrgios.

Outside of the 'Big Four', Wawrinka, the French Open champion, has made the semi-finals in New York on just one occasion in 10 visits.

Defending champion Marin Cilic faces the task of proving he's not a one Slam wonder.

The 26-year-old world number nine only started his season in March after suffering a shoulder injury and has yet to make a final this year.

World number four Nishikori became the first Asian man to reach a Grand Slam final in New York last year.

But despite winning in Washington and making the semi-finals in Montreal, the 25-year-old Japanese was forced to skip Cincinnati with a left hip injury.