At six of the last 10 Grand Slam tournaments, a woman has reached her first major singles final. All six first-time finalists lost the match, four of them in straight sets while winning no more than six games. Five then lost their first match at the next tournament. None has reached another major final since. Four of them failed to reach the quarterfinals at the next major they played. Three have fallen out of the Top 10 in the rankings.Breakthrough performances have been followed by letdowns.The most promising of the six players is Simona Halep. She came the closest to winning her major final debut, taking 15 games off Maria Sharapova at the French Open in June. Halep followed that by reaching the semifinal at Wimbledon the next month. And she enters the U.S. Open — which began this week — ranked No. 2 in the world. Yet she doesn’t look likely to reach the final in Flushing, New York. She won just two matches at warm-up tournaments, and Halep dropped the first set to unranked Danielle Rose Collins (the U.S. college singles champ) before coming back to win her opening match Monday.“Every day we have to work to reach the top and to stay there, because it’s more difficult to stay there than to reach it,” Halep said at a news conference after her win.It’s a bit early to declare the most recent first-time finalist a letdown; Eugenie Bouchard hasn’t gotten a chance to play another major since reaching the Wimbledon final this summer. On Tuesday, she begins her U.S. Open against Olga Govortsova. Early returns for Bouchard aren’t good, though: She’s won just one match in three tournaments since getting routed by Petra Kvitova in the Wimbledon final.Like the current group of young contenders, Kvitova didn’t immediately back up her breakthrough performance. She won Wimbledon in 2011, at age 21, in her first major final. Then she lost three of her next five matches, including her first-round match at the U.S. Open. But she won two tournaments and the Fed Cup later that summer, and Wimbledon this summer. She has been a regular in the Top 10 since reaching her first major final.Victoria Azarenka followed shortly after Kvitova and was more consistently successful. She reached her first major final at the Australian Open in 2012, at age 22, and won it — routing Maria Sharapova, as Kvitova had done the previous summer at Wimbledon. Then Azarenka won the next two tournaments she played and held the No. 1 ranking for much of the next year, including during her successful defense of her Australian Open title the next year.It’s natural that an athlete who is playing her first major final against a player who has been there before would be an underdog. And it’d be unfair to expect the player to repeat her performance at the next major, rather than regressing a bit to the mean. Plus, the women who have broken through recently are young and have time to return to the sport’s most prominent matches.Among the six most recent first-time major finalists, Sara Errani was the oldest at the time of her breakthrough. She had just turned 25 when she reached the 2012 French Open final, relatively young in the aging sport of tennis. Four of the others were younger than 24 when they reached their first Grand Slam final. But only Bouchard was younger at her first breakthrough than Kvitova and Azarenka were.