Karolina Pliskova continued to settle in to her new role as world number one by reaching the Rogers Cup quarter-finals when Japanese teenager Naomi Osaka was forced to retire with an abdominal injury on Thursday.
The 19-year-old qualifier, ranked 50th in the world, was giving Pliskova a mighty battle, winning a second set tiebreak, but after being broken to open the third she then called for the trainer and moments later retired trailing 6-2 6-7(4) 1-0.
Romanian second seed Simona Halep and sixth-seeded former-world number one Caroline Wozniacki needed no help reaching the last eight as they steamrollered past overwhelmed opponents.
Halep signalled that she is well prepared to successfully defend her Rogers title with a 59-minute 6-1 6-0 demolition of the 26th-ranked Barbora Strycova.
The Czech had never taken a set from Halep in three previous meetings and did not come close on a sweltering centre court as the Romanian converted five consecutive break chances.
In a battle of former Rogers Cup champions, a ruthless Wozniacki showed no mercy as she stormed past her good friend and 10th-seeded Pole Agnieszka Radwanska 6-3 6-1 in 78 minutes.
The only hiccup in an otherwise polished effort came when the Dane was up 5-0 in the second and squandered three match points.
The slip, however, only delayed the inevitable, the Dane closing out the match at the next opportunity with a stinging forehand winner.
"We've known each other since we were five and six years old so there are no secrets out there," said Wozniacki, who will now face Pliskova for a place in the semi-finals. "It is always unfortunate to play a good friend in such an early round but at the same time it is part of the game."
Pliskova, playing her first tournament since a shock second round defeat at Wimbledon, had shown signs of rust in a ragged opening win over Anastasia Pavlyuchenkova but looked more like a world number one as she swept four straight games to take the first set from Osaka.
But the Japanese would not be rattled, matching the lanky Czech shot-for-shot in a gripping second set then dominating the tiebreak 7-4 to level the match.
During the changeover Osaka told her coach that she was feeling discomfort and was told to withdraw if the pain persisted and avoid the risk of further injury.
Osaka did just that, calling for the trainer after dropping serve to open the third set.
"She played well, especially in the second set she was fighting pretty hard, much better than the first," said Pliskova, contesting her first event as the top ranked player.
"I was just trying to stay in the game, the first game of the third set was very tough I didn't even know she was injured."
Czech Lucie Safarova also made the last eight by outlasting Russia's Ekaterina Makarova 6-3 6-7(3) 6-2, while France's Caroline Garcia beat 18-year-old American Catherine Bellis 6-4 6-2.