Williams, one short of Margaret Court’s record of 24, lost in straight sets to Canada’s Bianca Andreescu in the US Open final on Saturday night.
“Serena will be looking at it, 44 per cent of first serves in, that’s not good enough,” said Barker in a BBC radio interview.
“She said afterwards she doesn’t know what happens and the real Serena doesn’t show up in the finals and that’s what she’s got to find the answer to.
“But maybe the pressure of the record is just too much.”
Williams has now lost in her last four Grand Slam finals and Barker, who won the French Open in 1976, feels the American could be running out of time.
Barker said: “There are players out there who aren’t afraid of her anymore – that’s the difference.
“She had this aura about her before, she was so dominant in the finals, nobody could get near her, she played the big points so well.
“But (on Saturday night) it was the youngster who played the big points better.”
Williams, who gave birth to her daughter, Alexis, in September 2017, won her last Grand Slam singles title at the Australian Open in 2017.
She has since lost in finals to Angelique Kerber (Wimbledon 2018), Naomi Osaka (US Open 2018), Simona Halep (Wimbledon 2019) and Andreescu (US Open 2019).
“She turns 38 later this month,” Barker added. “She is an incredible athlete and she’s been in four Grand Slam finals since she gave birth to her daughter.
“So she’s still right up there, she’s still doing well. If she’s still motivated and wanting to do it, I think she will carry on.”
Andreescu was congratulated by Canadian Prime Minister Justin Trudeau after her stunning victory.
As well becoming the first woman to triumph at Flushing Meadows on her main draw debut, the 19-year-old from Mississauga, near Toronto, is also the first Canadian to win a grand slam title.
Prime minister Justin Trudeau tweeted: “Congratulations. You’ve made history and made a whole country very proud.”
The Duchess of Sussex was in Williams’s box to cheer her close friend towards what was widely expected to be a record-equalling 24th grand slam title.
But the woman appearing in her first major final dealt with the occasion far better than the woman playing in her 33rd.
Williams’s first serve deserted her, with just a wretched 44 per cent of them hitting the spot. Having been broken just three times all fortnight, she dropped serve six times in the final, including the first game and the last.
The 37-year-old is nothing if not a fighter, though. Andreescu had championship point at 5-1 in the second but Williams, roared on by a vociferous crowd, hauled herself back to 5-5.
Andrescu put her fingers in her ears at one point, later admitting: “I couldn’t hear myself think”.
At the changeover at 6-5, Andreescu composed herself in the chair before holding her nerve to fashion two more match points and belting the second past the six-time champion for a remarkable victory.
“I told myself to put the goddamn ball inside the court,” she said. “I wanted to win the first point to show her that I am in it to win it. It was really, really loud. But I guess that’s what makes this tournament so special.
“I just tried to stay as composed as I could.”
Williams was understandably frustrated with her performance.
“Bianca obviously played well. At the same time it’s inexcusable for me to play at that level,” she admitted.