Andy Roddick has leapt to the defence of Serena Williams, claiming his fellow American remains the best player in the world with nothing left to prove.

Last year, Williams won the first three majors to raise hopes of a calendar Grand Slam, only for those ambitions to be dashed by a US Open semi-final defeat to Roberta Vinci.

In 2016, Williams has progressed to the finals of both the Australian and French Opens, but faltered at the last as she lost out to Angelique Kerber and Garbine Muguruza respectively.

Muguruza's triumph at Roland Garros prompted some talk that the dominant era of 21-time major champion Williams, 34, was over, but former world number one and 2003 US Open champion Roddick strongly rejected those claims.

"People who refer to this as a rough patch for Serena, we should all be so lucky to have our rough patch end at number one in the world," he told Omnisport.

"I think she's a victim of her own success; she's playing against the shadows of history now, not just against the current crop of players.

"We're not that far removed from her playing for the calendar-year Grand Slam. She's still great. She's still the best in the world.

"Other women might think they have a shot at her a little bit more on a day-to-day basis right now, but I've known her since we were kids, we're great friends, and she'll only come back harder when faced with a defeat.

"She's been dealing with uphill battles her entire life, so it's not going to be anything new for her. She has nothing to prove at this point, she's just playing for record books."