Andreescu, 19, extended Williams's wait for a record-equalling 24th Major crown with an impressive 6-3 7-5 victory in New York on Sunday (AEST).
But just like Naomi Osaka a year ago, when the Japanese star beat Williams in a controversial final, Andreescu was apologetic to what was a raucous crowd supporting the American great at Flushing Meadows.
Asked about the toughest aspect of her win in an on-court interview, Andreescu said: "Definitely the crowd.
"I know you guys wanted Serena to win, so I'm so sorry. Obviously it was expected for Serena to fight back. She's done that so many times in the past. That's why she's a true champion on and off the court.
"I just tried my best to block everything out. The last game wasn't easy – she started serving way, way better too. Balls were going all over the place. I'm just glad with how I managed really."
Andreescu managed to close out her first Major title despite Williams reeling off four consecutive games from 5-1 down in the second set.
The Canadian paid tribute to Williams and was delighted with her own ability to deal with the enormity of the situation.
"It's so hard to explain in words. I'm just beyond grateful and truly blessed. I've worked really, really hard for this moment. I can't complain," Andreescu said.
"This year has been a dream come true. Now, being able to play on this stage, against Serena, a true legend of the sport, is amazing.
"Oh, man. It wasn't easy at all. I tried to prepare my best, like I do for every other match. I tried to step on the court and tried not to focus on who I'm playing. That's really easy to say but I'm really proud of how I dealt with everything."