MASON, Ohio -- Going small paid off big for Venus Williams.
Tuesday didn't go as well for sister Serena.
On a day when back problems forced the younger Williams to withdraw before her second-round match, Venus Williams reached the third round of the Western & Southern Open with a 6-3, 3-6, 7-6 (4) upset of defending champion Kiki Bertens.
Her key? Not going for it.
"I just tried not to go too big because I can go so big, and I have a lot of power, and it's not always easy to control it,'' the 39-year-old Williams said. "So I'm trying to play smart instead of going hog wild, which is extremely easy to do.''
Venus, a seven-time Grand Slam champion, is the oldest woman in the draw. She has made her deepest Cincinnati run since she reached the 2012 semifinals.
"It was just a second round, though,'' Venus said. "For me, it's just round by round. I have a third round to play. I'm ready for the next round.''
Bertens, seeded fifth, rallied from deficits of 3-0 and 5-2 in the third set to force a tiebreaker. The 2-hour, 17-minute match ended when she sailed a forehand wide.
"I didn't do the things I was doing well coming back -- playing aggressive, going for my shots,'' Bertens said of the tiebreaker. "I didn't do that any more in the tiebreak, so there she could play her game again, which she did pretty good, I would say.''
Two-time Grand Slam-champion Petra Kvitova looked to have been caught off-stride after a rain delay, and she lost to Maria Sakkari 6-4, 2-6, 6-3.
Including Serena Williams's withdrawal, Kvitova was one of six seeded women to withdraw or be eliminated on Tuesday in this US Open tuneup. Thirteenth-seeded Angelique Kerber was knocked out by unseeded Anett Kontaveit 7-6 (7), 6-2, and 15th-seeded Qiang Wang lost to Su-Wei Hsieh 6-3, 6-4.
Two-time Australian Open champion Victoria Azarenka, who won Cincinnati in 2013, reached the second round after 12th-seeded Belinda Bencic retired because of an injured left foot while trailing 6-4, 1-0.
Madison Keys, the 16th seed, pulled off a 6-7 (4), 7-6 (5), 6-4 win over two-time Grand Slam winner and 2017 Cincinnati champion Garbine Muguruza.
The Associated Press contributed to this report.