WATCH the WTA LIVE in Australia on beIN SPORTS | Subscribe here and take advantage of our 7-day FREE TRIAL

Mboko was playing in her second-round singles match at Queen's but was forced to withdraw in the second set after slipping on the court and injuring her knee. 

The 19 year-old had lost the opening set 6-2 but was 4-3 up in the second before the incident occurred, and though she was able to walk off the court, she looked visibly upset. 

Mboko's injury came just 24 hours after she paired up with Williams in a 7-6 (7-2) 6-2 victory over third-seeded duo Nicole Melichar and Erin Routliffe at the Andy Murray Arena. 

That marked Williams's first competitive match since the US Open in 2022, with the pair due to face Leylah Fernandez and Laura Siegemund in the last eight. 

Elsewhere at Queen's, Amanda Anisimova sailed past Siegemund as she returned to the grass for the first time since her heavy Wimbledon final defeat last year.

Anisimova was beaten 6-0 6-0 by Iga Swiatek at the All England Club 12 months ago, but looked back to her best during a 6-1 6-3 triumph over Siegemund.

"It was an incredible year for me last year," said Anisimova, who will face compatriot Iva Jovic for a spot in the semi-finals. 

"A lot of highs, a lot of low moments where there were some tough lessons I had to learn, but those are the ones that shaped me as a person and an athlete.

"I'm trying to carry that into this year – everything that I learned."

At the Libema Open, fifth seed Anastasia Potapova was forced to come from a set down to beat wildcard Suzan Lamens 6-7 (5-7) 6-4 6-4 in three hours and 28 minutes. 

But in an all-American affair, Emma Navarro suffered a first-round exit at the hands of Caty McNally, who emerged a 4-6 6-0 6-4 victor. 

Williams said she thoroughly enjoyed her return to action, but her first shot at a comeback looks set to be short-lived, with her attention now potentially turning to the doubles at the Berlin Open. 

She is yet to reveal whether she will compete at Wimbledon, though one player that is certainly loving life back on the grass courts in London is Anisimova, who is aiming to go one better at Queen's after losing last year's final to Tatjana Maria. 

Anisimova fired two aces and won 87 percent of her first-serve points, but she will face a stern test against Jovic in what will be the first meeting between the pair.