Two straight nights, two grueling, seesaw battles.

Serena Williams took another step on her comeback injury and illness Friday, fighting off another determined opponent at the Rogers Cup.

The 29-year-old American booked her spot in the semifinals with a hard-fought 4-6, 6-3, 6-2 victory over Lucie Safarova of the Czech Republic.

The match stretched 2 hours, 4 four minutes a night after Williams had to battle back from a set deficit to beat China's Zheng Jie, also a two-plus hour match.

Good thing the 13-time Grand Slam champ upped her fitness regimen as part of her bid to come back from being sidelined for the better part of a year after her victory at Wimbledon in 2010.

"I just decided if I can be fit, that can be a new level in my game, because I've always been — I think — a halfway decent player," Williams said. "So I thought, 'OK, what haven't I done? I've never really been fit.' I'm still not super fit, I always have cream sodas at night. But I've been doing some running. I hate running, but I've been doing more running, distance running."

Williams will face No. 4-seeded Victoria Azarenka of Belarus on Saturday. Azarenka, the highest-seeded player remaining in the women's draw, breezed by unheralded Galina Voskoboeva of Kazakhstan 6-1, 6-2 earlier in the day.

Tenth-seeded Samantha Stosur of Australia will meet 13th-seeded Agnieszka Radwanska of Poland in the other semifinal. Stosur downed Italy's Roberta Vinci 6-4, 6-1, and Radwanska defeated No. 11 Andrea Petkovic of Germany 6-4, 6-3.

Williams' victory surely prompted sighs of relief from tournament organizers Friday night, the end of a long week that had seen the star power all but gone with 11 of 16 seeded players losing. Williams is unseeded after nearly a year away from the game, but is easily the biggest star remaining.

Williams is playing just her fourth tournament since she was injured shortly after winning her fourth Wimbledon at All England Club last summer.

A few days after that victory, she cut her foot on glass at a restaurant in Germany, which would require two operations. She spent 10 weeks in a cast and 10 weeks in a walking boot. Then she was diagnosed in February with blood clots in her lung.

She didn't return to the practice court until April.

"I definitely feel like I'm coming back from a layoff, it trips me out that this is only my fourth tournament. A lot of people are still expecting me to be at the top level and I expect it even more," she said.

Regaining her confidence, she said, has been the most difficult part of her return to the world stage.

"I had to play for confidence. I could go out there and no matter what I'm ranked, people still expect me to win sometimes and do really well. I was just not there, just trying to get there. Once I hit Stanford, I was there man, my prayers were answered," said Williams, who won the Stanford Classic late last month.

She struggled with her forehand, and Safarova's left-handed serve, in the first set, but poured it on the second. And by the time she broke Safarova's serve to go up 3-2 in the third, the momentum belonged to Williams, who fired eight aces on the night.

"My forehand is by far my favorite, and once I punched myself in the head I started hitting them way better in the last few games, I just had to get over what I was going through and so I came back. That was good," Williams said.

Williams had the Rexall Centre crowd chuckling after the match when she disclosed her plans for the evening had she not won. She had planned to go to sing karaoke.

"Air Supply ... " she said. "I'm in a real '80s mood right now."

Azarenka's victory over Voskoboeva ended a coming-out party for the 135th-ranked Kazakh. Voskoboeva had upset No. 9 Marion Bartoli, Flavia Pennetta and fifth-seeded Russian star Maria Sharapova.

"It's one week," Azarenka said. "If she can do that on a consistent basis, she'll be up in the top 10."

The 22-year-old Azarenka — who's yet to drop a set in the tournament and has lost just six games — broke Voskoboeva's serve six times. Voskoboeva showed her frustration by firing a ball that narrowly missed hitting a ball boy.

Meanwhile, the 11th-ranked Stosur broke Vinci's serve while leading 5-4 in the first set to grab the edge.

The Australian then poured it on against the 22nd-ranked Italian in the second to move into the semis in convincing fashion.

"Semifinals, you've got to believe that anything in possible," she said. "Having said that, from semifinals to winning the tournament, there's a still a long way to go, but I think in this position you've got to. If you don't, then it's probably a little bit silly."

In a battle of versatility over power, the 22-year-old Radwanska took 1 hour, 41 minutes to dispatch Petkovic, winning the final long rally when the 23-year-old German fired a shot into the net.

Radwanska is battling a shoulder injury, which she summed up to as "too much tennis." The right-handed Polish player wore a patch the size of a saucer taped over her right shoulder.

"Playing every day is not very helpful with my shoulder," Radwanska said. "Still I can handle it, and for sure I will do my best (Saturday)."