Top-seeded Garbiñe Muguruza of Spain withdrew on Tuesday from the Brisbane International tennis tournament during her opening match due to muscle cramps in her right leg.
Muguruza, age 24, was unable to finish the match against Serbia's Aleksandra Krunic, 7- 5, 6-7(3), 2-1, requiring medical assistance on the court and leaving in tears.
"I start to feel my calves were cramping. And I continued to think they might go away, and then they were increasing, increasing," the world No. 2 said.
"I cannot believe it. I don't know. It's a shame because I always come here excited the first tournament, and this one was bad luck, I guess," Muguruza added.
The Spaniard could have regained her position as world No. 1 had she not been forced to retire from the tournament in this match, which lasted two hours and 16 minutes.
Krunic qualified for the quarterfinals, where she is set to play Anastasija Sevastova of Latvia, who defeated Sorana Cirstea of Romania 6-2, 6-1.
Muguruza had also been registered in the doubles tournament, pairing with fellow Spaniard Carla Suarez.
The Czech Republic's Karolina Pliskova, seeded No. 2, is now the favorite to win the tournament, and is due to take on Catherine Bellis of the United States as her first opponent.