For the second straight year, Serena Williams’s bid to make history at the U.S. Open was derailed in a semifinal upset. Last year, Williams was seeking to complete the first calendar-year Grand Slam since Steffi Graf in 1988 and to tie Graf’s Open-era record of 22 career major titles when she was defeated by No. 43 Roberta Vinci. On Thursday night, Williams was two match wins from passing Graf in career major titles, but, hampered by an injury to her left knee, she lost to No. 11 Karolina Pliskova.Williams was ranked No. 1 heading into both matches. And according to win probabilities based on our tennis Elo ratings, both results were big surprises: She had a 97 percent chance of beating Vinci and an 87 percent chance against Pliskova.1In this article, our estimates of win probability heading into matches at this year’s U.S. Open use a blend of players’ overall Elo ratings and Elo ratings based only on hard-court matches, according to the methods described in our article about our tournament forecast. For earlier matches, we’re using probabilities based on overall Elo ratings for both players. That’s not unusual for Williams, who has left majors in two ways over the past five years, each one happening about as often as the other: Either as the champion (9 of 20) or as the loser of a big upset (11 of 20). She has had at least an 85 percent chance of winning all 11 of the matches she has lost at majors since 2012 (her first year playing the majors in her 30s). Those 11 losses are among the 14 most surprising of her career at majors: In other words, her 14 Slam losses for which she entered the matches with the highest Elo-based win probabilities.The series of surprise losses is the result of two factors:1) Williams is losing more matches she should win, perhaps a reflection of increased susceptibility in her 30s to injury or wear during a tournament. She has won 89 percent of Slam matches since 2012 in which she had an 85 percent chance or better, compared with a 96 percent win percentage in the same type of matches before 2012.2In both time periods, the average of her win probability in these matches was 95 percent.2) Against a field with fewer all time-greats than she faced earlier in her career, Williams is playing a lot more matches as a big favorite, giving her more opportunities to be upset: She has had at least an 85 percent chance of winning in 88 percent of her major matches since the start of 2012, compared with 64 percent earlier.