Angelique Kerber has wished Serena Williams "health and happiness for the best journey in the world" after it was confirmed the 23-time grand slam champion is expecting her first child and will miss the rest of the 2017 season.
Williams appeared to initially announce her pregnancy via a Snapchat post on Wednesday, which saw a picture of the American accompanied by the caption "20 weeks".
The post was soon deleted, but Williams' publicist duly confirmed to the WTA that the 35-year-old is expecting a baby "this fall" and intends to return to competitive action in 2018.
In a message on her official Twitter account on Thursday, Kerber congratulated her rival at the top of the rankings.
"@serenawilliams !! You will be such an amazing mother and I wish you and your family health and happiness for the best journey in the world," wrote Kerber.
Former world number one Caroline Wozniacki, a close friend of Williams, also welcomed the news.
"'Auntie Caro' sounds pretty good to me," the Dane wrote on Twitter. "Everyone around me getting pregnant! What a beautiful time."
Andy Roddick added: "There's gonna be a baby GOAT [Greatest of all Time] ... We are so happy, and we know @serenawilliams will be a great mother."
WTA CEO Steve Simon offered his congratulations in a statement that read: "We're excited for Serena. This is wonderful news and we wish her the very best.
"We'll look forward to seeing her when she's back competing again."