David Ferrer will have an MRI scan to determine the severity of the thigh injury that forced him to withdraw from the Mexican Open.
Ferrer, top seed in the men's draw at Acapulco, led Thursday's quarterfinal against South Africa's Kevin Anderson by winning the first set 6-2. He strained his left leg and retired at 4-2 down in the second.
Earlier this month, the 31-year-old Spaniard won the ATP Copa Claro title for a third year running and had hoped to build on that success in Acapulco.
However, he now awaits results that will determine the extent of his forced layoff.
"I'm very sad because I was playing great," said Ferrer, who is No. 4 in world rankings.
Ferrer's exit leaves Andy Murray as the big favorite to win the Mexican Open 500. Murray overcame a poor start to beat France's Gilles Simon 1-6, 7-6 (4), 6-2.
"It's tough to win matches when you get down like that, I've been struggling with my service in the first two matches," said Murray, who will play against the winner of the match between Latvia's Ernests Gulbis and Bulgarian Grigor Dimitrov.
Murray has beaten Simon 12 consecutive times.
Anderson will next play Ukraine's Alexandr Dolgopolov, who defeated Ivo Karlovic of Croatia, 6-4, 7-6 (4).
On the women's side, top-seeded Dominika Cibulkova of Slovakia defeated New Zealand's Marina Erakovic 6-4, 7-5 to set up semifinal against China's Shuai Zhang.
Zhang, the eighth seed, ousted Ajla Tomljanovic of Croatia 6-1, 5-7, 6-2.
Canada's Eugenie Bouchard lost in a surprise 3-6, 6-4, 6-1 loss to France's Caroline Garcia.
Garcia, 89th in the WTA rankings, will play in the semifinals against American Christina McHale, who defeated third-seeded Kai Kanepi from Estonia 6-1, 2-6, 6-4.
Contributing: OmniSport, The Associated Press