Keys admitted that nerves got the better of her during a 6 - 2 6 - 4 loss to Simona Halep .
Keys became the first American woman to debut in the top 10 since Serena Williams in 1999 earlier this season .
Keys , who became the first American woman to debut in the top 10 since Serena Williams in 1999 earlier this season , admitted that nerves got the better of her during a 6 - 2 6 - 4 loss to Simona Halep .