Pete Sampras believes it would be foolish to write off Roger Federer's chances of winning an 18th major at the US Open despite the Swiss having just turned 34.

Federer's last Grand Slam crown came at Wimbledon in 2012, and his three-year drought is the worst of his career.

But Sampras, a 14-time Grand Slam title winner, insists the veteran can collect a sixth US Open to add to the five he won between 2004-2008 when the season's final major gets underway in New York on Aug 31.