After waiting out a storm that suspended play for an hour, Sharapova eliminated the No. 7 seed to advance past the third round of the Canadian tournament for the first time.
It was another positive step for the 49th-ranked Sharapova, who returned in May from shoulder injury that kept her out of action for nine months.
"I thought it was a step up, definitely," Sharapova said. "I had to (play) against somebody that I've had trouble against in the past who I usually play three sets against. It was good to finish the match in two, but I really had no choice. I had to pick it up."
Zvonareva won four straight games in the second set before Sharapova recovered to force the tiebreaker, which she won easily. Sharapova will next face No. 14. Agnieszka Radwanska of Poland.
"I think this is definitely a great result," Sharapova said. "I've already played against three great opponents and I'm going to play another opponent tomorrow and you just hope that with each match you can raise your level and play better."
Williams had a much easier time, beating Ukraine's Alona Bondarenko 6-1, 6-4, two days after sister Venus Williams fell to Kateryna Bondarenko.
Serena Williams will face Lucie Safarova of the Czech Republic next. The second-ranked Williams is the highest-seeded player left in the tournament after No. 1 Dinara Safina lost Wednesday.
Fifth-ranked Jelena Jankovic slugged out an 1-6, 6-3, 7-5 win over Belgium's Kim Clijsters in the late match. Jankovic, fresh from winning the Cincinnati Open on Sunday, needed 2 hours to beat Clijsters, who returned last week after retiring two years ago to start a family.
Clijsters didn't look like a player who had spent time away from the court. The 26-year-old scored four break points off Jankovic and will take that momentum to the U.S. Open after receiving a wild card for the tournament Thursday night.
At first, it looked like another easy day for Williams. After cruising through the first set, she appeared ready for the next round when Bondarenko found her swing in the second set.
But Williams took a breath, relaxed, and won three straight games to move on.
It was a minor hiccup in Williams' quest for a second Rogers Cup title that she chalked up to a momentary loss in composure.
"I was just trying to be perfect and then I just couldn't get my serve in and got frustrated," Williams said. "You know, I've been practicing a lot on my serve, it just hasn't been well, so I think in any case it can be frustrating."
Earlier, Elena Dementieva rallied past Shahar Peer for a 6-1, 1-6, 6-4 victory. Dementieva will face Samantha Stosur, who swept Virginie Razzano 6-3, 6-1.
Aravane Rezai couldn't repeat after her upset of Safina, falling 6-3, 6-4 to Alisa Kleybanova; Safarova beat China's Zheng Jie 7-6 (3), 6-4; and Radwanska downed Kateryna Bondarenko 7-5, 6-3..
Williams has rarely been challenged at the Rexall Centre. She easily beat Yaroslava Shvedova on Wednesday and may not face a seeded player until the semifinals.
However, she knows she has to keep her cool if she's going to make it that far.
"I get really emotional on the court," she said. "Sometimes I'm really into the game too much, and I've just got to (remain calm). I can't help myself when I do that, I think it's just something natural and hopefully it helps me. I think towards the end I was able to relax and just calm down and then just try to stay positive more than anything."
She admitted she was disappointed in Thursday's performance after a strong start to the tournament.
"I thought that if I could keep up that form, I'm on my way into glory and today I didn't play as well so hopefully I can get it back," Williams said.
Williams needed just over an hour to beat Bondarenko, hitting three aces while committing three double faults. Williams, who will compete at the U.S. Open, said she is focused on winning the Rogers Cup for the first time since 2001.
"I'm just trying to play well," she said. "There's a huge gap so I'm just trying to do what I can and stay alive here. This is my first focus."
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