WHAT HAPPENED: Flavia Pennetta’s body language said all there was to know about a first set in which nothing went right. The tour veteran was able to shake a lackluster start and rally past the relentless Petra Cetkovska, 1-6, 6-1, 6-4, punching a ticket to the round of 16.

Cetkovska went from the brights lights of Arthur Ashe Stadium to the cozier confines of Court 17. Expectations increased after her thrilling three-set, do-or-die upset over No. 4 seed Caroline Wozniacki, but the odds were stacked against the 30-year-old Czech to reach her first Grand Slam round of 16. Yet again, she came out of the locker room with an all-or-nothing attitude and played a dominant first set in which she converted three of four break chances and won 77 percent of her first serves.

“It was really looking bad,” Pennetta said of the first set in the on-court interview after the match. “In the end, I was starting to play my game.”

Playing at a torrid, disruptive pace in the first set, the latter two sets leave questions of whether Cetkovska hit a physical wall playing in her third match this week. The world No. 149 played in only three pro matches all season after a slow recovery from hip surgery.

Pennetta fought back in the second set by working away from Cetkovska’s potent cross-court forehand, a shot she used to paint the lines with pinpoint accuracy and which made up the majority of her 17 first-set winners.

Both players broke serve to start the third set. As the crowded swelled to standing-room-only proportions, Pennetta made sure to quiet those cheering for an upset. With Pennetta serving at 3-2, she fought off three break chances, tallying some of her fastest serves of the match. Cetkovska rallied back to win the game, but Pennetta broke back at 3-3. When she needed her serve most, Pennetta found it, racking up six aces in the final set.

For the Czech, who saved four match points just two nights ago, another US Open miracle fell short. Cetkovska was able to save one match point, but the No. 26 seed went into survival mode to pull out the victory.

WHAT IT MEANS: Cetkovska has a lot to be proud of after a strong showing this week.

When it was suggested in the on-court interview that Pennetta came out groggy in the first set, the 33-year-old Italian joked that “one eye was open, one eye was closed” watching her boyfriend Fabio Fognini’s late-night (into early morning) match against Rafael Nadal.

Pennetta, who hasn’t gotten past the round of 32 in a hard-court tournament this summer, is into the round of 16 at the US Open. She’ll be aiming for her sixth US Open quarterfinals berth when she takes on 2011 US Open champion Samantha Stosur.

THE QUESTION: What was more impressive: Cetkovska’s run, or Pennetta’s resilience?