Ajla Tomljanovic sent Serena Williams into almost certain retirement on Friday when she knocked the tennis legend out of the US Open before hailing her as "the greatest of all time. Period."

The American 23-time Grand Slam title winner, and six-time champion in New York, lost her third round clash 7-5, 6-7 (4/7), 6-1.

"I'm feeling really sorry just because I love Serena just as much as you guys do. What she's done for me, for the sport of tennis, is incredible," said Australia's Tomljanovic.

"I never thought I'd have a chance to play her in her last match when I remember watching her in all those finals, so this is a surreal moment for me."

It was a nervy conclusion for the 29-year-old Tomljanovic who sealed the deal on a sixth match point after more than three hours on court.

"I just thought she would beat me, so the pressure wasn't on me. Even to the last point, I knew that she's in a position to win even when she's down 5-1," added Tomljanovic.

"That's just who she is and she's the greatest of all time. Period."