SAN FRANCISCO - Dominika Cibulkova, who was playing her first match since reaching the quarter-finals of Wimbledon, defeated Urszula Radwanska 7-6 (7/3), 6-3 in her opening match at the WTA Stanford tournament on Thursday.

The second seeded Slovak was one of the last among the top seeds to begin play at the California hardcourt tournament. Cibulkova jumped out to a 5-2 lead in the first set but Radwanska roared back to force the tiebreaker before Cibulkova prevailed in just over an hour.

The pair traded breaks in the second set before Cibulkova was able to clinch it in the ninth game.

"At the right time, I was focused and I didn't panic though I lost a few games I should have won. My forehand was working and I knew I had to go for that, and I had to play smart today because I knew she was playing really well," said the world No. 12 Cibulkova.

Cibulkova will face Japan's Misaki Doi in the quarter-finals. She is hoping that a strong performance in Stanford will vault her back into the top 10 in the world rankings.

In other matches, 17-year-old Catherine Bellis beat Sachia Vickery 4-6, 6-4, 6-3 in a battle between two players ranked 203rd and 204th in the world. The American teen next faces 36-year-old top seed Venus Williams for a berth in the semi-finals when the two duke it out on Friday night.

Bellis is no stranger to upset victories as she first burst onto the scene as a 15-year-old in the 2014 US Open when she stunned Cibulkova in three sets, 6-1, 4-6, 6-4. Six months later she reached the third round of the Miami event where she faced Serena Williams for the first time.

Friday's contest will be her first time playing seven-time Grand Slam winner Venus Williams. Bellis was born five years after Venus played her first career WTA Tour match at this event in 1994.

Isner, Monfils cruise into Washington quarters: US top seed John Isner and French second seed Gael Monfils breezed into the ATP and WTA Washington Open quarter-finals Thursday with impressive serving performances in straight-set triumphs.

Isner blasted 15 aces in downing Cypriot 15th seed Marcos Baghdatis 7-6 (7/3), 6-2 while Monfils fired 12 aces and won 22 of 23 first-serve points in defeating Croatian teen 16th seed Borna Coric 6-2, 6-3. Neither winner faced a break point. "When you win most of your points with the serve, you can focus on your shots and you feel less pressure," Monfils said. "I've been serving much better this year. I've changed some motions with my legs. I serve a lot and it's working so I enjoy it. I trust my serve and the court is fast so I can switch from my defense to attack."

Monfils, the 2011 Washington runner-up, will next meet US eighth seed Sam Querrey, who ousted Ukraine 11th seed Alexandr Dolgopolov 7-6 (7/5), 6-3.

Three-time Washington runner-up Isner next faces US fifth seed Steve Johnson, who ousted American qualifier Ryan Harrison 6-4, 6-4. Johnson won his first ATP crown last month at Nottingham.

"He turned a corner and he has carried that confidence over," Isner said of Johnson. "He's playing very well. This is an extremely tough match."

No American has won the Washington title since Andy Roddick in 2007. "Still the number one is John," Monfils said. "I will get to the final if I can. I've got to focus on my game."

Ball-bashing Ivo Karlovic, the 37-year-old Croatian who won last week at Newport to become the ATP's oldest singles champion since 1979, smashed 21 aces in dumping Australian third seed Bernard Tomic 7-6 (7/4), 6-3.

US sixth seed Jack Sock downed Britain's Daniel Evans 6-1, 7-5, to book a last-eight date with Karlovic, the only men's quarter-finalist not among the top eight seeds. "It's a good tournament for big servers," Isner said. "Myself, Karlovic is still alive, Jack and Stevie. A big serve and a forehand." Sock hit only 44 percent of his first serves but won 23 of 25 first-serve points. "I was able to 1-2 punch, serve and forehand and get a few points that way," Sock said.

French fourth seed Benoit Paire outlasted Luxembourg's 14th-seeded Gilles Muller 6-2, 3-6, 7-6 (7/4) and will play Germany's Alexander Zverev for a semi-final spot.

Zverev, 19, matched his Washington run last year by beating Tunisia's Malek Jaziri 6-2, 5-7, 6-2. Risa Ozaki, a 22-year-old from Japan who lost five of her prior six WTA matches before this week, outlasted Britain's Naomi Broady 6-1, 5-7, 6-4, to reach her first tour quarter-final, where she will face Kazak sixth seed Yulia Putintseva.

Ozaki ousted defending champion and second seed Sloane Stephens in her opening match. American Lauren Davis upset Puerto Rican third seed Monica Puig 6-4, 6-2 to reach a quarter-final against Italy's Camila Giorgi, who advanced by walkover when Austrian Tamira Paszek withdrew with an upper respiratory infection.

French fourth seed Kristina Mladenovic beat Germany's Sabine Lisicki 6-3, 6-3, and next plays Belgian seventh seed Yanina Wickmayer.

Australian top seed Samantha Stosur meets US wildcard Jessica Pegula in Friday's other women's quarter-final.