Rising teenager Emma Raducanu came out on the winning side of a generational clash against tennis icon Serena Williams with a 6-4 6-0 win in their first-round meeting at the Cincinnati Open on Tuesday.

After a sluggish start, the 23-time Grand Slam champion finally gave the supportive sold-out crowd something to cheer about when she crushed back-to-back aces to cut Raducanu's lead to 4-3.

But the English reigning U.S. Open champion fired an ace of her own to snag the first set and followed that up with a break of serve to open the second.

Raducanu rolled from there, smacking an unreturnable serve on match point to end their first career meeting.

"I think we all just need to honor Serena and her amazing career," Raducanu said in an on-court interview.

"I'm so grateful for the experience of getting to play her and for our careers to have crossed over. Everything she has achieved is so inspirational, and it was a true honor to get to share the court with her."

Williams, 40, was world number one and had already won four major titles when Raducanu was born in November 2002.

Williams won her last major in 2017 while pregnant with her daughter Olympia, who was in attendance.

With the loss, Williams has just one professional tournament remaining before she drops the curtain on her historic career – the U.S. Open, which begins August 29.

Osaka out

Earlier in the day, Naomi Osaka's U.S. Open preparations suffered another setback as the former world number one was swept aside 6-4 7-5 by China's Zhang Shuai.

It was only Osaka's third tournament back from an Achilles injury, and it has been a stuttering return to action for the twice U.S. Open champion, who also exited in the opening round in Toronto last week, retiring with lower back pain.

For Zhang, doubles champion in Cincinnati last year, it was her first singles win at the event since 2014.

"Naomi, she is amazing, but I don't know she is maybe not really feeling good today," said Zhang. "But for sure today – not her best today."

Gauff hurt, Venus Williams falls

American teenager Coco Gauff rolled her left ankle late in the first set in her match against qualifier Marie Bouzkova, and she eventually retired from the match while trailing 7-5 1-0.

The newly crowned world number one in doubles will look to recover ahead of the U.S. Open where she will hope to compete for a first Grand Slam title.

This year's major at Flushing Meadows starts August 29.

Venus Williams was defeated by 14th seed Karolina Pliskova.

Venus Williams was bidding for her first win over a top-20 ranked opponent since overcoming Kiki Bertens in Cincinnati three years ago.