And with Russian Zvonareva gunning for revenge, third seed Clijsters knows she will have to improve on today’s display if she wants to make it through to a meeting with either Li Na or Caroline Wozniacki.

She said: “Everything has to be better: serving, returning, the unforced errors. I am in the semi-finals but I don’t feel like I have played my best tennis.

“I feel like I am hanging in there, working hard to win my points. And sometimes that’s probably even more of an achievement than winning your matches very easily.”

Clijsters realises a meeting with Zvonareva, who beat her at Wimbledon en route to a final defeat to Serena Williams, represents a major step up in class from her previous matches but she believes she can raise her level at the business end of the tournament.

“She is a player who will be very consistent throughout the whole match,” she added. “She won’t really mix her game up tremendously, she’ll always give you the same kind of thing. I think she does that well.

“At the US Open I don’t think she played her best tennis in the final and I was able to take advantage of that. It will be tough, there will be a lot of long rallies.

“But throughout the years I have always been able to lift my game when it is necessary.”

Clijsters cruised through the first set against Radwanska, breaking four times to the Pole’s twice, and took the match on a second set tie-break after suffering a wobble which saw her opponent hit back from 4-2 down to win three games on the trot.

“I fought well and stayed really focused,” she added. “But I felt physically a bit tired and heavy out there today. I just didn’t feel quite right.”

Zvonareva ended the run of Petra Kvitova 6-2 6-4 in the first match of the day on Rod Laver Arena.

It looked like she would be in for an easy day when she raced through the first set, breaking three times, before establishing a 3-0 lead in the second.

But an incident in the crowd, which saw a lady treated by medical staff, followed by another interruption while nearby cannons greeted Australia Day threw Zvonareva off track.

Kvitova, who knocked out home favourite Sam Stosur in round three, hit back with two breaks to go 4-3 up before Zvonareva regained her composure.

She broke for 4-4 and then again to claim the match when the Czech lashed a forehand long.

“I had to stay aggressive,” Zvonareva said afterwards. “She is such an aggressive player herself and you don’t want her to keep going for her shots.”

The US Open setback was Zvonareva’s only defeat to Clijsters in four meetings in 2010 and the Russian is hoping she can again get the better of the 27-year-old.

“I will definitely try to take positives from those matches when I beat her and try to think over again why I lost so quickly in the final,” added Zvonareva, who believes she has improved as a player after twice coming so close to breaking her grand slam duck.

“Those experiences have helped me a lot. I think I am more mature and will try to use that to my advantage. You learn from it and deal with things much better.”