|US Open 2019|
|Venue: Flushing Meadows, New York Dates: 26 Aug – 8 Sep|
|Coverage: Live text and BBC Radio 5 Live Sports Extra commentary on selected matches on the BBC Sport website and app. Click here for Live Guide.|
Defeat means American Williams is still chasing a record-equalling 24th major.
“I believe I could have just been more Serena today,” said Williams, a six-time champion at Flushing Meadows.
“I honestly don’t think Serena showed up. I have to kind of figure out how to get her to show up in Grand Slam finals.
“It’s inexcusable for me to play at that level.”
The 15th seed held match point at 5-2 in the second set before Williams staged an attempted comeback, winning five successive games.
“I was just thinking, honestly at that point, wow, this is terrible,” said Williams. “Like you got to play better. I have to do better.
“I just couldn’t go down like that, so I just wanted to play a little bit better.”
Andreescu broke Williams’ serve for the fourth time of the set to wrap up the win, making her the first Canadian to win a Grand Slam title and the first teenager to do so since Maria Sharapova at the 2006 US Open.
“I’m not necessarily chasing a record. I’m just trying to win Grand Slams,” Williams said.
“It’s definitely frustrating. But for the most part I just am still here. I’m still doing what I can do.”
‘I’m so close, yet so far away’
Williams said her successive falls at the final hurdle were “truly frustrating” but vowed to keep going.
“Kerber doesn’t count because I was exhausted,” she said. “My baby was eight months, and that’s tough.
“But all of it honestly, truly is super frustrating. I’m so close, so close, so close, yet so far away.
“I guess I’ve got to keep going if I want to be a professional Tennis player. And I’ve just got to just keep fighting through it.
“I definitely did better than I did against Halep.”