New York, New York - Serena Williams says this is the most free she has felt on a tennis court in over 20 years after shocking world number two Anett Kontaveit 7-6 (7-4) 2-6 6-2 to advance to the third round of the US Open.

Williams (40) found another gear in the deciding set as she rode the wave of momentum that came from the raucous, sold-out Arthur Ashe Stadium crowd.

While Williams rose to the occasion, Kontaveit folded, posting three winners and 11 unforced errors in the third set, allowing the all-time great to secure the victory with a break-to-love.

On the court after the match – after a recorded video message from Oprah Winfrey also aired after Monday's win over Danka Kovinic – Williams said while everyone else may have been surprised by the result, she still has supreme confidence in her abilities.

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"Well, I'm a pretty good player," she said playfully. "You know, this is what I do best."

"I love a challenge. I love rising to the challenge. Yeah, I haven't played many matches, but I've been practicing really well."

"In my last few matches, it just wasn't coming together. I'm like, this isn't me. The last couple matches here in New York, it's really come together."

Williams admitted that she thought to herself "this could be it" after dropping the second set, but highlighted the fact that this is the first time in over 20 years that she does not feel like she has a target on her back.

"Honestly, I'm just looking at [this run] as a bonus," she said. "I don't have anything to prove. I don't have anything to win. I have absolutely nothing to lose.

"Honestly, I never get to play like this since '98 really. Literally, I've had an 'X' on my back since '99. It's kind of fun."

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"I really enjoy just coming out and enjoying it. It's been a long time since I've been able to do that."

Lastly, Williams talked about what it will be like to step back out onto the doubles court with sister Venus Williams on Friday night.

"I'm so excited for doubles," she said. "It's like, it's been so long. We got to play again, we got to bring the reunion back.

"I'm excited to play with Venus, get some more practice – because I could use the matches."