ISTANBUL, Turkey—Last year, after contracting her umpteenth virus of the season, defending champion Petra Kvitova was forced to pull out of the WTA Championships after only one match—her first career defeat to Agnieszka Radwanska.

This year, the tall Czech lefty enters the event with only a cold and in much better spirits. That was clear in her 6-4, 6-4 victory over the Radwanska, her fifth in six attempts.

Kvitova broke Radwanska in the opening game when the Pole missed one of her standard plays, a low sidespin forehand approach shot, which she dumped into the net.

Since she won Wimbledon in 2011, Kvitova has been consistently been making noise about becoming a serve-and-volleyer. Given how effective she has been indoors—she owns a 66-21 record—she should have enough confidence to at least to try and make the transition this week.

But she held off against Radwanska, preferring to smack a big serve, a huge groundie, and then, only if she had Radwanska way off the court, come in to net. Headstrong net charges are not her forte yet.

After being broken to open the match, Radwanska settled down in her own service games, but she could not get a whiff during Kvitova’s, who teed off on cross-court and inside-out forehands and was also very aggressive with her cross-court backhand. Her slice serve out wide was working, suffocating the little magician, who was unable to string the points out long enough. Kvitova moved the ball around the service box very adeptly and served the first set out at love with two clean screamers from inside the baseline that Radwanska had no chance to run down.

Radwanska is a delight to watch when she is creating new patterns on the fly, but she can grow frustrated when she can’t get into points against the top power players. Not only is she 1-5 against Kvitova, she is also 0-7 versus Serena and 3-12 against Victoria Azarenka.

After falling behind a break to start the second set, Radwanska finally managed to claw her way into one of Kvtiova’s service games, breaking right back when the 23-year-old missed a big second serve down the tee. But Kvitova didn't lose any confidence, ripping a series of outright return-of-serve winners to again break Radwanska and lead 2-1.

From there it seemed like it was just a matter of time before Kvitova would rain down some toxic serves and close it out. She did, but not before revealing her enigmatic side. She threw in some—on this day—uncharacteristic unforced errors in the final game, which forced her to fight off three break points before winning the contest on her third match point; that after successfully challenging a call on a Radwanska forehand that landed just behind the baseline.

With the way that Williams dismantled another lefthander, Angelique Kerber, Kvitova will be put under tremendous pressure when the two match up in the Red Group later this week. But Kvitova did push Williams to 7-5 in the third set in a defeat in Doha in February, and said it was first time she really believed she could beat the great American. If she plays as well as she did against Radwanska, at the very least, Serena should have something to worry about.