World number one Dinara Safina joined former champion Maria Sharapova as a third round casualty on a day of shocks at the US Open.

Russian Safina, still searching for a first grand slam win, was beaten by world number 70 Petra Kvitova of the Czech Republic after having been made to wait three hours to start her night match and then having the court switched.

That was a direct result of Sharapova's three-hour marathon defeat to American wild card Melanie Oudin and Andy Roddick's fifth set tie-break loss to John Isner but it did Kvitova no harm as she completed a third set tie-break win 6-4 2-6 7-6 (7-5).

With the day session matches running behind schedule on Ashe the top seed saw her match switched to Armstrong with preference given on the main show court to American 21st seed James Blake's match with number 14 Tommy Robredo.

Whether the long wait to begin her match and the change of courts had an adverse affect or not, Safina was quickly in trouble against the world number 70, just as she had been in squeaking through her first two matches at Flushing Meadows.

Despite her poor form, the draw had been kind to the world number one with all the seeds in her quarter eliminated giving her a potentially clear path to the semi-finals.

Safina, though, seemed determined to do things the hard way and in an opening set that saw six breaks of serve, the Russian offered her Czech opponent eight break points, Kvitova taking four of them on the way to taking a 1-0 lead. There were more service woes in the second set when she was broken by Kvitova in the first game.

At 2-0 down, though, she showed some grit to save three break points and then broke serve herself on the way to pulling level at one set all. Despite her success, Safina was still uptight and berating herself for errors, which were still very much in evidence at the start of the final set.

Kvitova was dominating the top seed's second serve and again broke the Russian in the opening game although Safina did hold the third game to get on the board at 2-1 down.

Safina broke back in the sixth game to tie the final set up at 3-3. Kvitova was still right in the match but playing as if it had already got away from her and looking extremely tired. She sent a straightforward overhead into the net, throwing her racquet to the floor in disgust and promptly lost an opportunity to break again as Safina celebrated going 4-3 up.

At 5-5, Safina could still not shake off her serving problems, falling behind in the game with her fifth double fault of the set, ninth of the match, but benefiting from some tired play from Kvitova to take a 6-5 lead over the 19-year-old.

The next game saw Safina save three match points and holding her nerve to take the match to a deciding tie-break.