Pete Sampras thinks Serena Williams is feeling the effects of so many years at the top but said she is not finished yet.
Pete Sampras does not envisage Serena Williams experiencing a fall from grace and believes a lack of motivation is the main reason behind her inability to win a record-equalling 22nd grand slam title.
World number one Williams was beaten by Garbine Muguruza in the French Open final on Sunday, having lost to Angelique Kerber in an Australian Open showdown back in January
Williams sensationally failed to win the US Open on home soil last year when Roberta Vinci dumped her out at the semi-final stage, having looked set to complete a first calendar Grand Slam.
The legendary American still needs one major victory to draw level with Steffi Graf's Open Era record tally of 22 and fellow-great Sampras thinks a lack of drive after so many years at the top is a major factor for his compatriot.
He told Omnisport: "I think her biggest obstacle is motivation. She's 34, she's won over 20 majors. She's been around for a long time, she's getting a little banged up and these girls [Williams' rivals] are getting younger and better.
"I still see her as the best player in the world, it's her motivation, it's her drive. Because when you've been up there for so long it's tough to year in, year out to have that every day.
"She's done a lot for the game, she's a great player but when you get older it gets tougher."
Fourteen-time grand-slam champion Sampras expects his compatriot to put up a strong challenge to enter the history books on the grass at Wimbledon in a few weeks' time.
He said: "She's been around for maybe 16-17 years and you're going to have your ups and downs, but I look at Wimbledon and she's going to be right there at the end. It's a tough loss [at Roland Garros], but she'll be fine moving on.
"She's an all-time great, she's done a lot for the game and every now and then you're going to lose."
He added: "I don't see her slipping; it's more battling herself and her motivation."