In the absence of injured 2010 champion Serena Williams, third seed Clijsters is favourite to win her first Australian Open crown this year even if she has not been at her best since the first round 6-0 6-0 drubbing of Dinara Safina.
The 27-year-old Belgian hustled and bustled her way through the opening set in less than half an hour and took a 4-2 lead in the second before allowing Radwanska back into the contest with a slew of unforced errors.
The 12th seed won four straight games to put herself on the brink of levelling the match when she served for the set at 5-4, just as an aerobatics team started flying low and loud over the Rod Laver Arena in celebration of the national holiday.
Both knew that this was a key point in the match and slugged it out through 12 lengthy points but Radwanska blew her chance and Clijsters re-gathered herself, going on to win the tiebreak 7-4 with a clubbed backhand.
"I think the planes took me up higher," Clijsters laughed, before adding: "It was a little scary for me; they looked like they were very low.
"I just physically felt a little bit tired and heavy out there today," Clijsters added. "There were rallies where I felt good, I was hitting the ball, there were rallies when I didn't feel right in the position where I should be at."
The three-time U.S. Open champion will probably have to raise her game for the semi-final against Zvonareva, whose win over Kvitova was also disrupted by Australia Day celebrations when a 21-gun salute boomed out over the city.
Wearing a black ribbon on her visor in memory of the victims of Monday's bombing at Moscow airport, Zvonareva was a class above the Czech.
An aggressive Zvonareva clipped through the first set in 29 minutes and was 3-0 up in the second when Kvitova got onto the front foot, slugging her way back into the contest at 3-3.
Zvonareva, however, is determined to prove she can win a grand slam this year and ultimately went through a comfortable winner when Kvitova overcooked a forehand after 75 minutes.
(flandersnews and reuters)