With defeat to Raducanu, Serena running out of time to find form for US Open

Serena Williams' farewell tour suffered another bump in the road on Tuesday with a 6-4 6-0 opening round loss to Emma Raducanu at the Cincinnati Open, one of the last tune-up events before the final Grand Slam of the American great's career.

Williams has just one professional tournament remaining before the US Open, where she will have one last chance to tie Margaret Court's record of 24 Grand Slam titles.

After a sluggish start in the WTA 1000 event in Cincinnati, Williams gave the sold-out crowd something to cheer about in the first set when she crushed back-to-back aces to cut reigning US Open champion Raducanu's lead to 4-3.

But the British 19-year-old fired back 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 on from there, smacking an unreturnable serve on match point to end their first career meeting, setting up a clash against two-time former major winner Victoria Azarenka.

Williams waved to the crowd before walking off the court and did not speak to the media later.

"I think we all just need to honour Serena and her amazing career," Raducanu said on court.

"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 honour to get to share the court with her."

Williams, who will turn 41 next month, was world number one and had already won four major titles when Raducanu was born in November 2002. She won her last major in 2017 while pregnant with her daughter Olympia, who was in attendance.