What happened: Serena Williams has withdrawn from her doubles match alongside her sister Venus after sustaining a knee injury. The injury occurred during her singles defeat at Wimbledon, cutting short her return to the All England Club.

Why it matters: The withdrawal ends the sisters' planned doubles campaign announced in June, one of the marquee storylines of Serena's comeback that began at the HSBC Championships earlier this summer. A knee issue now clouds the next steps of a return that had already produced a singles wild card and her first Wimbledon match in years.

What to watch: Watch for updates on the severity of the knee injury and whether it affects the remainder of Serena's comeback schedule this summer.