World number one Serena Williams cruised into the quarter-finals of the Toyota Princess Cup on Wednesday with a 6-0 6-2 win over Austrian Patricia Wartusch.

The 2000 Princess Cup champion broke three times and took just 20 minutes to tie up the first set, in her first match since winning the US Open.

Although Wartusch managed to hold serve twice in the second set, Williams was never troubled and the match was over in just 50 minutes.

Afterwards Williams admitted that she had found it difficult to motivate herself since her straight sets win over sister Venus at Flushing Meadows.

She said: "I was talking to Martina Navratilova today about how after she won all her Grand Slams, the toughest match was the first match after.

"I felt it in practice. I haven't even had a week off and I found it very hard to be motivated.

"Martina said she experienced the same thing and that helped me realise it was normal."

Williams will play either Ai Sugiyama of Japan or Australian Nicole Pratt in Friday's quarter-final.

Russian Elena Likhovtseva joined Williams in the next round after upsetting eighth seed Arantxa Sanchez-Vicario 6-2 6-7 6-4 in a match lasting two hours, 15 minutes.

In first round matches, sixth seed Tatiana Panova overcame Henrieta Nagyova 6-2 1-6 6-4, while seventh seed Tamarine Tanasugarn beat Adriana Serra Zanetti 6-2 7-6.

Elsewhere, Nicole Pratt beat Gala Leon Garcia 6-3 7-6, while Lina Krasnoroutskaya of Russia beat Croatia's Silvija Talaja 6-3 6-4 to set up a second-round clash with defending champion Jelena Dokic.

Russian qualifier Svetlana Kuznetsova was 5-0 up in the first set against Tina Pisnik before the Slovenian was forced to retire with back pain.

Kuznetsova will play third seed Kim Clijsters in the second round on Thursday.

Second round results from the Princess Cup:

First round
N Pratt (Aus) bt G Leon Garcia (Spa) 6-3 7-6 (7-4)
L Krasnoroutskaya (Rus) bt S Talaja (Cro) 6-3 6-4
S Kuznetsova (Rus) bt T Pisnik (Slo) 5-0, retired
C Fernandez (Arg) bt P Mandula (Hun) 7-5, 6-2
(6) T Panova (Rus) bt H Nagyova (Svk) 6-2 1-6 6-4
(7) T Tanasugarn (Tha) bt A Serra Zanetti (Ita) 6-2 7-6 (7-3)
A Molik (Aus) bt C Torrens Valero (Spa) 6-4 6-2

Second round
E Likhovtseva (Rus) bt (8) A Sanchez-Vicario (Spa) 6-2 6-7 (1-7) 6-4
(1) S Williams (USA) bt P Wartusch (Aut) 6-0 6-2