Serena Williams thegrio.com
(Photo: Elsa/Getty Images)

Two days after Serena Williams defeated sister Venus Williams in a U.S. Open showdown, she hit 18 aces and beat Kaia Kanepi 6-0, 4-6, 6-3 to advance to the quarterfinals for the 15th time.

Tennis Smash reported that Williams was unstoppable on Sunday as she swept through the opening set against Kanepi in a record-breaking 18 minutes.

“It wasn’t an easy match at all,” said Williams after the match. She last won the US Open title in 2014.

“Kaia knows how to play and has a lot of big wins. She has had a great career,” she added. “We always have tough matches, so I am just happy to get through. She was hitting such big returns so I wanted to hit bigger second serves.”

Here’s one of the more impressive returns of the match:

The tennis champ also noted:

“I like it when players challenge me to do that. I think what I have improved most is my fitness. Coming back from having a baby is really hard.”

READ MORE: Colin Kaepernick and Eric Reid receive much love at U.S. Open

Kanepi defeated No. 1 Simona Halep in the first round, si.com reported, and according to the outlet, her match against Williams was the fastest set thus far in this year’s U.S. Open.

One enthusiastic sports fan @PrashantSport hit up Twitter to praise the star athlete, writing: “After a flawless 18 minute opening set, things got a little complicated. Serena Williams weathered the Kaia Kanepi storm and got through to the QFs! The GOAT!”

Williams will move on to play No. 8 seed Karolina Pliskova in the quarterfinals. As noted by Yahoo, the two previously faced off at the U.S. Open, where the Czech player defeated the Compton native 2-6, 6-7 in the 2016 semifinals.

READ MORE: Serena Williams beats sister Venus Williams to advance in U.S. Open

She topped her sister Venus in the third round of the 2018 U.S. Open on Friday, winning 6–1, 6–2. Now, Williams is eyeing her 24th Grand Slam singles title and seventh U.S. Open.