Serena Williams US Open semi-final defeat a 'monumental shock'Williams, 33, was two wins from completing the calendar Grand Slam but lost 2-6 6-4 6-4 to unseeded Italian Roberta Vinci in the semi-finals.
"This is monumental. It's a shocker," Austin told BBC Sport.
"This is one of the biggest upsets in the history of tennis, because of what was on the line."
Williams went into Friday's semi-final on a 33-match winning streak in Grand Slams stretching back to Wimbledon 2014.
She had hoped to match Steffi Graf's 1988 sweep off all four majors in the same year, and was up against a player she had beaten four times without dropping a set.
Vinci, 32, is ranked 43 in the world and was playing in her first Grand Slam semi-final.
"Roberta Vinci had a losing record coming into the US Open and somehow got hot," added Austin.
|The lady that beats Serena, Roberta Vinci|
"Things went her way, the draw opened up, Genie Bouchard hit her head and couldn't play that match [in round four]. It's just the way things happen sometimes.
"That's why they put the net up every day. You've got to go out and handle the nerves.
"I always said Serena's biggest opponent coming into this year's US Open was going to be her own nerves."
However, angry fans are already blaming Serena’s ‘boyfriend’, hiphop star Drake for her woes….
#BlameDrake: Serena Williams' US Open exit is Canadian rapper's fault, say angry fansSerena Williams's loss in the US Open semi-finals has been blamed on the Canadian rapper – and the tennis player's rumoured boyfriend – Drake.
Rumours the pair are dating turned negative after Williams lost to Roberta Vinci, Time reports.
Had she won, she would have been the first player since 1988 to win all four major tennis tournaments in a calendar year.
Twitter users were quick to blame Drake for her loss, using the hashtag #BlameDrake.
The rapper has a poor reputation when it comes to supporting his favourite sports teams.
He was appointed the global ambassador for the Toronto Raptors in 2013, only for the team to be wiped out that season, according to the Sydney Morning Herald.
He was then given a cease-and-desist letter from the University of Kentucky basketball team, advising him to stop attending their matches.