Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) The US Open women's singles draw is down to four -- and all of them are American. On Wednesday afternoon at Arthur Ashe Stadium in Flushing, New York, 20th-seeded CoCo Vandeweghe upset No. 1 Karolina Pliskova, last year's US Open finalist, 7-6(4), 6-3. In the night session, No. 15 Madison Keys cruised against Kaia Kanepi of Estonia, 6-3, 6-3. "I was real nervous today," Keys said on court as the last American to clinch her spot. "The car ride over, I was definitely feeling it. But I got out here, I felt really good." Vandeweghe and Keys join No. 9 Venus Williams and Sloane Stephens, who is unseeded, in the semifinals, which start Thursday. Williams, who won a dramatic, hard-fought match Tuesday against No. 13 Petra Kvitova, will face Stephens, who overcame No.16 Anastasija Sevastova. Both Williams and Stephens advanced by winning third-set tiebreaks.CoCo Vandeweghe and Madison Keys both won WednesdayThey join Venus Williams and Sloane Stephens in the semifinals 
 Question:This will be the second major semifinal for _, at age 22.

Madison Keys