Sloane Stephens loses to Andrea Petkovic at Mercury Insurance Open

It’s a matter of experience as much as talent, a battle in the brain as much as on the tennis court sometimes.

Sloane Stephens is 18 years old and has played only 10 WTA-level matches. Andrea Petkovic is 23 and has been a pro since 2006, and when you play against the best more often you learn things.

For example, Stephens made a crucial mistake in the first set, in the sixth game of her quarterfinal match Friday at the Mercury Insurance Open in Carlsbad, when she and Petkovic had traded service holds and the match seemed destined to stay competitive.

It was one wrong choice, Stephens said, that started her downward trend. She hit a ball cross-court. “I should have hit it down the line and come in. But I didn’t,” Stephens said.

From there Stephens had her serve broken. Then the first set disappeared and soon the match. Petkovic, seeded second, ended up winning the quarterfinal, 6-2, 6-1, in 66 minutes and it will be the German who will play third-seeded Agnieszka Radwanska in one semifinal Saturday.

Despite playing with a sore right shoulder and suggesting that her hopes of playing that match were “50-50. It’s hard to say if I can do it or not,” Radwanska beat eighth-seeded Daniela Hantuchova, 0-6, 6-4, 6-4.

Saturday’s other semifinal will feature top-seeded Vera Zvonareva, who is ranked third in the world, against fifth-seeded Ana Ivanovic.

Zvonareva, who wore a bright pink T-shirt after the match, played white hot tennis in the third set and withstood big-serving Sabine Lisicki in her 6-3, 3-6, 6-4 win.

Lisicki recorded a 123-mph first serve in the third set, but it was Zvonareva’s backcourt consistency that won out.

Ivanovic trailed in the first set, 4-1, against fourth-seeded Peng Shuai of China before calling for her new coach, Nigel Sears, to come out and counsel her. Sears’ advice to calm down and trust herself worked. Ivanovic won, 6-4, 6-2.

“He earned his money tonight,” Ivanovic said.

Petkovic said she felt Stephens’ inexperience mattered Friday.

“In the beginning, the first six games, it was a really tough battle. I was equaI,” Petkovic said. “I think after I made the break for the 4-2 lead, it was too much with her inexperience. But her ball is really, really heavy and she hits it so well. She has a great future in front of her.”

Ivanovic hopes she has a future in front of her as great as the past once was. The 23-year-old was ranked No. 1 once, after she won the French Open in 2008. Ivanovic was only 15-13 in matches this season before coming to Carlsbad, an inconsistency that caused her to take a new coach after she had lost in the third round at Wimbledon.

Now she is into the semifinals of this tournament for the first time and has matched her best result of the year.

diane.pucin@latimes.com

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