Updated

Well, she sure answered that question.

Anyone wondering how serious a threat Kim Clijsters might be at the U.S. Open after a two-year layoff got their answer Sunday, when the 2005 champion at Flushing Meadows toppled none other than No. 3 Venus Williams, 6-0, 0-6, 6-4 to advance to the quarterfinals.

Moving well, stinging shots with both forehand and backhand and matching every bit of Williams' power, the Belgian star offered a startling referendum on the state of her game.

It may have also said something about the true state of Williams' left knee, which she hurt in the opening round, but refused throughout to use as an excuse.

"It was unbelievable, I don't know what to say," Clijsters said. "It was such a weird match, especially those first two sets. But after I lost the second at 6-0, I said, let's start over and start a new match."

Indeed, the match began the way many Sundays do in Queens — with a couple of bagels. It took a grand total of 50 minutes, but both players regrouped from that bit of awkwardness and played some of the most compelling, solid tennis of the tournament so far.

Clijsters grabbed an early break for a 3-1 lead, helped by one of Williams' five double-faults to close it out, then served out the match, though it was anything but routine.

She fell behind 0-30 on her serve at 5-4, but just kept banging away. She got it to 30-40, the hit a shot deep into the corner that Williams couldn't handle, forced an error at deuce with another deep groundstroke, then skidded a service winner off the line on the backhand side for the win.

Her reaction was one not so much of surprise, as a smile that seemed to say "I told you so."

"I've been working really hard the last seven, eight months and I'm enjoying it," Clijsters said. "It's something that's really important for myself, as long as I can focus on tennis and have fun on the outside as well."

The Belgian star, who retired to start a family and hadn't seen Grand Slam action since 2007, re-established herself as one of the few who can move well enough and hit hard enough to hang with the Williams sisters.

She is trying to become the first unseeded player to reach the finals of the Open since Venus Williams did it in 1997.