NEW YORK, NY, USA - For the second year in a row, 23-time Grand Slam champion Serena Williams departs the US Open after coming up just short in the final against a young, determined opponent. 

In a stunning display of power and composure, 19-year-old Bianca Andreescu halted Serena’s quest for a record-tying 24th Grand Slam trophy in straight sets - making herself Canada’s first Grand Slam champion in singles in the process. 

With a bit of distance between her and her fourth consecutive defeat in a championship match, Serena admitted that she might be able to look back on these finals as a testament to her post-maternity will and strength. 

Read more: 'I've been dreaming of this moment' - Andreescu ascends to US Open title after Serena stunner

But tonight, the frustration at finishing the tournament with another runner-up trophy is still too fresh. 

“I feel like in 20 years, I definitely will be like, ‘Wow, that wasn't so bad,’” Serena said in her post-match press conference. “It's very hard right now in the moment to, like, take this and say, ‘It didn't work out for you today,’ 

“It's really hard right now to take that moment in and to say you did okay, because I don't believe I did. I believe I could have played better. I believe I could have done more. 

“I believe I could have just been more Serena today. 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.”

Serena had nothing but praise for her young opponent Andreescu, who played fearless tennis against the former World No.1, breaking Serena six times in two sets. The clash was a rematch of their Rogers Cup final in Toronto, where Andreescu was also crowned champion after a back injury forced Serena to retire.

“I felt like Bianca plays well under pressure,” Serena assessed. “She played really well at Indian Wells. She was definitely not a favorite there, but she goes out and she plays hard. She does what she does best, and that's move up to the ball, that's hit winners, that's play with a ton of intensity.

“I just feel like she did that really, really well today… She played really well, and she deserves this championship.” 

The frustration was palpable in Serena’s voice as she reflected on not being able to find her best tennis at the crucial moments - particularly on her serve. Tonight, Serena put just 44% of first serves in play, committed 33 unforced errors (58 errors total), including eight double faults.

Serena Williams and Bianca Andreescu share the podium after their US Open final. (Getty Images)

“That was obviously on my mind, like how do I play at a level like this in a final?” Serena said. “I think Bianca obviously played well. I think her returns make me play better and puts pressure on my serve. At the same time it's inexcusable for me to play at that level.”

For Serena, there is one positive that she’s been quick to take away after the 6-3, 7-5 defeat: in the second set, Serena saved championship points at 5-1, and battled back to level the score at 5-5. 

Despite ultimately coming up short, those signs of fight and grit are uplifting for the former World No.1 as she continues to seek her first post-materinity title.

“I honestly didn't play my best today,” she admitted. “I could have played better. That's the only solace that I can take right now.”

Serena Williams speaks to press after her defeat in the US Open final. (Jimmie48 Photography)

But even that comfort proves to be a double-edged sword: in the quarterfinals, Serena dropped just one game against No.18 seed Wang Qiang, and then dismantled the Elina Svitolina defense in the next round - both victories were a commanding display of power and controlled aggression against in-form opponents. 

Knowing that she has another level - one that she has even been able to tap into during tough battles this fortnight - makes the defeat in the final infinitely more disappointing. 

In photos: Story of the final: Bianca Andreescu crowned at US Open after Serena victory

“All of it honestly, truly is super frustrating,” Serena told press. “I'm, like, so close, so close, so close, yet so far away. I don't know what to say. 

“I guess I got to keep going if I want to be a professional tennis player. And I just got to just keep fighting through it.”