NEW YORK — Since Christina McHale won a first-set tiebreaker against her idol Serena Williams at Wimbledon last month, she’s played three tournaments and World Team Tennis to prepare for the U.S. Open.

Before considering a rematch, McHale needs to defeat first-round opponent Mona Barthel of Germany on Monday.

McHale hasn’t advanced past the third round of a Grand Slam, but her confidence grew after a 6-7 (7), 6-2, 6-4 loss to the eventual Wimbledon champion, who earned a record-tying 22nd major singles title.

“There were a lot of positives I took from my Serena match,” McHale said. “The first and most important for me being that I can compete with the best. That match has really motivated me to keep working and improving.”

McHale served well early and kept Williams off balance in winning the tiebreaker during the second-round match at Wimbledon. In the second set, McHale went up 40-15 on serve, with two chances to take a 3-0 lead.

But she double-faulted and Williams ran off 11 straight points. McHale also double-faulted on a game point and Williams attacked her second serves to break for 5-4.

It was a lesson in thought management for the 24-year-old McHale.

“You think about the big picture all of a sudden,” Billie Jean King said. “Your brain goes from one ball at a time to, ‘Oh crap, I’m up playing for 3-0 and I’m up a set already. I should win this.'”

McHale played nearly every day for two weeks in early August for the New York Empire in the World Team Tennis league co-founded by King. The tennis great knows it’s important to stay focused and relaxed during critical points.

“Serena talks about being in the zone,” King said. “Everybody understands that means stay in the present and be in the now. It sounds so easy. Before the point is how you tee it up and get ready. Seventy-five percent of the time in the match, you are not hitting a ball.”

McHale started playing tennis in Hong Kong, where she lived from age 3 to 8 and learned to speak Mandarin. She also was a competitive swimmer growing up in Englewood Cliffs, New Jersey. She turned pro in 2010, and last year, she moved to South Florida to train at the USTA National Tennis Center in Boca Raton.

Her older sister Lauren graduated from North Carolina, where she played tennis for the Tar Heels. Lauren is engaged to American tennis player Ryan Harrison.

Christina reached a career-high singles ranking of No. 24 in 2012. She’s coached by Jorge Todero, who has guided her the last five years. McHale has posted wins over former No. 1 Caroline Wozniacki and Grand Slam champions Victoria Azarenka, Petra Kvitova and Svetlana Kuznetsova.

“He’s gotten me to use my forehand more,” McHale said of the Argentine. “I try to hit as many forehands as I can, go to my strength in the pressure moments.”

She’s currently ranked 55th with a 31-20 record this year. Her first serve percentage is just 59 percent, with 64 percent of those points won. She’s had 119 aces and 136 double-faults.

After Wimbledon, McHale upset eighth-seeded Monica Niculescu at the Citi Open in Washington in mid-July. She then lost to Jessica Pegula, daughter of Buffalo Bills and Sabres owners Terry and Kim Pegula, 7-5, 6-2.

She bowed out in the second round in Montreal at the Rogers Cup, but joined Asia Muhammad to defeat the top-ranked doubles team of Martina Hingis and Sania Mirza 6-3, 6-4 in the quarterfinals before losing in the semifinals.

McHale, who recently reached the second round in Cincinnati, is among 19 American women in the U.S. Open. Next up is Barthel, who she beat in matchups in Seoul in 2014 and Madrid in 2015.

If McHale gets past her, she’ll play the winner of Roberta Vinci vs. Anna-Lena Friedsam. Vinci took out Williams in the semifinals of the U.S. Open last year, ending the quest for a calendar year Grand Slam.

Patrick McEnroe, who coached the Empire and McHale, calls her “a grinder.”

“She’s right there. I’ve always been impressed with her work ethic,” said McEnroe, who previously helped McHale while leading the USTA player developmental program. “From the first time I saw her, she’s really been committed to putting in the work to be the best she can be.”