Serena Williams' injured left knee forced her to withdraw Tuesday from the Internazionali BNL d'Italia ahead of her second-round showdown with sister Venus Williams.
"I must withdraw from the Italian Open due to pain in my left knee," the No. 10 seed and former world No. 1 said. "I will miss the fans and competition at one of my favorite tournaments. I'll be concentrating on rehab and look forward to seeing you all at the French Open and next year in Rome."
A four-time champion in Rome, Serena Williams is the winningest active player in the Italian capital with a 43-8 record.
Serena Williams withdrew from the Miami Open in March with the same injury. Venus moved to the third round in a walkover and will face either No. 7 seed Sloane Stephens or England's Johanna Konta.
During Tuesday's rain-interrupted final match, Victoria Azarenka of Belarus stunned No. 5 seed and two-time defending champion Elina Svitolina of Ukraine, 4-6, 6-1, 7-5.
No. 4 seed Karolina Pliskova cruised into the third round with a 6-3, 6-3 win against Ajla Tomljanovic of Australia in 71 minutes.
"It was a really good match for me because the conditions were tough," Pliskova said. "It was very windy and cold, so I think we were not really playing our best tennis. But I was a little bit better. I was serving well, so I managed to win."
The Czech player opened up a 4-1 lead in the first set and a 4-0 lead in the second. She had five aces and one double fault, compared to four doubles by Tomljanovic.
No. 8 seed Ashleigh Barty of Australia survived a 4-6, 6-3, 6-4 fight from Slovakia's Viktoria Kuzmova. Barty, who hasn't lost to a player outside the top 10 since September, tallied 33 winners, including 10 aces, and converted six of her 12 break points.
France's Alize Cornet delivered a 6-1, 6-4 upset against No. 9 seed Aryna Sabalenka of Belarus, who piled up 40 unforced errors over the course of one hour and 14 minutes.
"The whole thing for me was to be solid, to play my game, to fight until the end, and that's what I did," Cornet said. "I think it's a solid win overall."
Due to a calf injury, No. 11 seed Caroline Wozniacki of Denmark retired after dropping the first set 7-6 (5) against Danielle Collins.
Switzerland's Belinda Bencic rallied from a set and double-break down to beat No. 12 seed Anastasija Sevastova of Latvia, winning 2-6, 6-3, 6-2. Bencic won 92 percent of her first-service points in the final set, winning 17 straight points over one stretch.
No. 16 Julia Goerges of Germany posted a 6-3, 6-4 win over Hsieh Su-Wei of Taiwan.
Spain's Carla Suarez Navarro recorded the 500th WTA main draw victory of her career with a 6-4, 1-6, 6-3 victory against Ukrainian teenager Dayana Yastremska.
--Field Level Media