Second-seeded Serena Williams also advanced with a 6-3, 6-1 win over Daniela Hantuchova, losing just two points on her first serve and breaking Hantuchova five times to complete the win in 56 minutes.
Venus led 6-1, 0-1 when Ivanovic called it quits.
Ivanovic took a 10-minute break during the first game of the second set to have her left thigh taped by a trainer. She returned for two more points, but after hitting a service winner to take the game, she began crying as she walked to her chair and told the umpire she was retiring.
Venus, seeking her sixth Wimbledon title, took a 5-0 lead before Ivanovic won a game 27 minutes into the match. Former No. 1 Ivanovic hasn’t reached a Grand Slam quarterfinal since winning the 2008 French Open.
Serena is looking for her third Wimbledon title after winning here in 2002-03. She lost last year’s final to her sister.