Serena Williams endured another grand slam final nightmare as 19-year-old Canadian Bianca Andreescu rose to the occasion at the US Open.

Andreescu shot out of the blocks before surviving some late nerves to win 6-3 7-5 as Williams hopes of a record-tying 24th grand slam slipped away in front of close friend Meghan, Duchess of Sussex, who watched from her player box.

The 37-year-old Williams has lost all four major finals she has reached since returning from giving birth, including to Naomi Osaka in Flushing Meadows last year.

She has failed to even win a set in those four matches and remains one behind Australian Margaret Court's record for most Grand Slam trophies.

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"She does get tight but those four women have played the matches of their lives," tennis legend Chris Evert said in commentary for ESPN.

It was after the match in the press conference however where things took a frosty turn with one reporter questioning Williams' reaction to the loss.

The questions were a clear look at last year's US Open when Williams erupted at chair umpire Carlos Ramos and stole the limelight from Naomi Osaka's victory.

Reporter: "Your resolve and congratulations seemed genuine. Are these losses starting to have less impact on you?"

Williams: "I don't know how to answer that."

Reporter: "You congratuled your opponent."

Williams: "I always congratulate my opponent, yeah."

Reporter: "Last year you were excusing more yourself than today or not?"

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Williams: "I don't understand that question. Excusing more myself? Anyway, thank you."

It wasn't all frosty in the room however as an open and honest Williams admitted the showing was by far her worst of the tournament.

"I honestly don't think Serena showed up, and I have to figure out how to get her to show up in Grand Slam finals," she said.

"I love Bianca and she's a great girl but I think this is the worst match I've played all tournament."

Ranked 178th in the world at the start of the year, Andreescu became the first Canadian woman to win a Grand Slam title and the first teenager to win a major since Maria Sharapova in 2006.

She has not lost a match since March and is 8-0 against top 10 opponents in a stunning rise to the top of the sport. "This year has been a dream come true,'' Andreescu said on court. "I know you wanted Serena to win, so I'm so sorry.''

Andreescu made her intentions clear by breaking Williams in the first game of the match.

Both players held serve comfortably in the next few games before Williams finally earned a break point in the eighth game. Andreescu responded with an ace and held to move within one game of the first set.

She only needed one more opportunity, breaking Williams — whose first serve was far from where she needed it to be — for a second time.

Williams stumbled out of the blocks again in the second set, dropping her serve immediately to fall behind 2-0.

She rallied in the third game, finally making in-roads on Andreescu's serve, but it was a brief moment of hope as she dropped her next two service games to fall behind 5-1.

There were some understandable nerves for Andreescu as she attempted to serve out the match for the first time.

Williams fought off the first championship point she faced and broke to stay alive at 5-2.

She had the crowd on its feet after holding to make it 5-3 and Arthur Ashe was positively rocking when she broke to love to get it back on serve and then held to make it 5-5.

But Andreescu finally regained her composure to hold serve and then break again to win the match.

"I was fighting at that point to stay out there a little more,'' Williams said of her comeback. "The fans started cheering so hard it made me play a little bit better and fight a little bit more. If anyone could win this tournament outside of Venus, I'm happy it's Bianca."

Williams won the player box battle, with Markle, sister Venus Williams and Anna Wintour of Vogue cheering her on.