STANFORD, CALIF. — In the end, beating two Williams sisters in a row was too much to ask.  A day after downing Venus Williams in a grueling three-set quarterfinal, Germany’s Andrea Petkovic fell to Serena Williams 7-5, 6-0 in one hour, 17 minutes at the Bank of the West Classic.  For Williams, it marked her 12th straight win at Stanford — a streak that stretches back to 2011 — and sent her into her third final here in four years.

“For me, it was honestly the most awkward situation,” confessed Petkovic, who had hoped to be the first player to defeat both Venus and Serena at the same event since Jelena Jankovic pulled off the feat in Rome in 2010.  “I literally grew up with both of them, idolized them and admired them.  They were just so different and so powerful, so much charisma.  It was just something different in tennis.  Playing one of them is a lot, but playing two of them in a row was so weird.  It was just like a video game.”

Serving to stay in the match trailing 6-5 in the evenly played first set, Petkovic, a first-time semifinalist here, appeared to tighten up and quickly found herself down 0-40.  Although she would battle back to set up a game point opportunity, she couldn’t hold it and a long forehand later gave her opponent the stanza.  The German swatted a ball against the back of the court in frustration.

“I was really fighting at that point,” said Williams, who will spend her 200th non-consecutive week at No. 1.  “I thought it would be a great opportunity for me to break because I could start the second set serving.”

Any momentum that Petkovic had seemed to leave the Taube Tennis Center in a hurry, and Williams, who totaled six aces and won 84 percent of her first-serve points, cruised through the second set all but unopposed.  Though she managed to fight off eight of 12 break-point opportunities, the No. 18-ranked Petkovic was under pressure on her serve for much of the afternoon.  Serena, meanwhile, nullified the only break point she faced.

“She really didn’t give me a lot of looks at her second serve,” said Petkovic.  “When she puts in a lot of first serves, it’s very difficult.”

Of course, the match may have been over before it even began.  Petkovic pointed out that it’s hard to warm up when your surefire Hall of Fame opponent’s resume is being announced to the crowd.

“The umpire goes, ‘Serena Williams, 500 Grand Slam titles, 700 times winning the gold medal!’  I’m there, ‘Four titles.’  Yes!” said the 26-year-old, her tongue-in-cheek scenario bringing laughter in the pressroom.  “It doesn’t help when you’re going to serve for the first time and then you hear all the Grand Slams they have won, and all the gold medals, 478 titles on the WTA Tour.”