Serena Williams' hopes of claiming a third consecutive title in the build-up to the US Open were shattered when she was forced to withdraw from the Western & Southern Open in Cincinnati with a toe injury.
Williams aggravated the injury in her first round win over Lucie Hradecka and pulled out of her scheduled match against 10th seed Samantha Stosur, who advanced to round three.
Meanwhile, world number one Caroline Wozniacki was stunned by US wild card and world number 76 Christina McHale, who took three minutes shy of two hours to defeat the Dane 6-4 7-5.
The top players in the men's draw had an easier time of things, number one seed Novak Djokovic easing past second-round opponent Ryan Harrison 6-2 6-4 and number two Rafael Nadal also easing to a 6-4 7-5 win over Julien Benneteau to help erase memories of his shock loss to Ivan Dodig in Montreal last week.
Fourth seed Andy Murray recorded his first win since Wimbledon with a 6-1 6-4 defeat of an off-colour David Nalbandian, while fifth seed David Ferrer was taken to three sets before seeing off Grigor Dimitrov 4-6 6-1 7-5.
Dodig was forced to retire when he trailed 4-6 6-3 4-0 to sixth seed Gael Monfils, while other seeded winners included Mardy Fish, Richard Gasquet and Nicolas Almagro.
Qualifier Radek Stepanek continued his fine recent form by beating Andrey Golubev in straight sets and there were also wins for Philipp Kohlschreiber, Alex Bogomolov Jr and Fernando Verdasco.
Elsewhere in the women's competition, second seed Vera Zvonareva eased past fellow Russian Ekaterina Makarova 6-3 6-0.
Jelena Jankovic, Francesca Schiavone, Andrea Petkovic, Svetlana Kuznetsova and Shuai Peng were the other seeded players to make it through.
Ana Ivanovic, the 15th seed, went down in straight sets to Nadia Petrova and 17th seed Yanina Wickmayer was beaten by Petra Martic. Daniela Hantuchova and Shahar Peer also progressed with straight sets victories over Flavia Pennetta and Pauline Parmentier, respectively.