World number one Ana Ivanovic battled her way into the third round of the Montreal Cup to keep her nose in front in the fight for the top ranking on Wednesday.
Number three Maria Sharapova, however, will take no further part after withdrawing from the tournament with a shoulder injury following her win over Poland's Marta Domachowska.
Playing her first event since a shock third-round exit at Wimbledon, Ivanovic arrived with her top ranking under serious threat from fellow Serb Jelena Jankovic and Sharapova.
Ivanovic opened her hardcourt campaign with a laboured 6-3, 4-6, 6-3 win over Czech teenager Petra Kvitova while Jankovic, who had a chance to nudge her Davis Cup team mate out of the top spot last week in Los Angeles, dispatched local hope Aleksandra Wozniak 6-0, 6-4 to reach the last 16.
Sharapova, also back in action for the first time since Wimbledon, had a less than impressive hardcourt debut, serving up 17 double faults in a three-hour 7-5, 5-7, 6-2 win over Domachowska.
The Russian twice required treatment on a right shoulder that has bothered her throughout the year, and said after the match she was withdrawing.
"Bad news, because I'm withdrawing," Sharapova told reporters. "It was pretty obvious out there. Coming into this tournament, I wasn't quite sure if I was going to be able to compete. Tomorrow I will go for an MRI.
"It's tough to go on court and not be close to even 50 percent. I'm too good of a player to go out there and try to fight through something that I think can eventually become something serious."
CANADIAN WOE
The Canadian hardcourt event has never ranked among Sharapova's favourites, the Russian winning just one match in two previous visits and had not played in the event since 2004.
Ivanovic, meanwhile, showed signs of rust in her first competitive match in nearly a month, taken to break point six times in her opening two service games, none of which Kvitova could convert.
The French Open champion served nine double faults, surrendering the second set, before rebounding to sweep the opening five games of third to effectively seal the win.
"It wasn't the best match I played," said Ivanovic, the Canadian champion in 2006. "It was a tough one. It was the first match since Wimbledon so in the beginning obviously I struggled to find my rhythm a little bit.
"You have to win even if you don't play your best tennis. It's never easy."
Victories for Ivanovic and Jankovic meant they became the first players to qualify for the year-ending $4.4 million Sony Ericsson championships in Doha in November.