World number one Simona Halep won a three-hour, seven-minute marathon to defeat Anastasia Pavlyunchenkova to reach the third round in Montreal on Thursday but faces a return to the court later in the day to take on Venus Williams at the rain-hit US Open tune-up.
Top seed Halep was 2-4 down in the third set against Russia's Pavlyuchenkova before she rallied to capture five of the last six games to claim the second-round tie.
"It was a great match, a great fight. She played unbelievable and was fighting until the end," said Halep, who was playing her first match since Wimbledon, in an on-court interview.
"I won the match, and I'm really happy that I could fight until the end. With this support, it's impossible to give up."
The French Open champion added: "I'm little bit dead. It's a long time since I played my last match at Wimbledon. I will try to do my best to recover and fight next match."
Pavlyuchenkova's challenge was eventually undone by 65 unforced errors.
Williams also has to play twice in one day after Wednesday's rain-lashed programme.
The veteran 13th-seeded American moved into a clash against Halep for a spot in the quarter-finals with a 7-6 (7/4), 6-4 victory against Romania's Sorana Cirstea.
Williams, 38, committed 30 unforced errors and seven double faults while Cirstea, the 2013 runner-up, was unable to convert eight of her 11 break points.
US Open champion Sloane Stephens reached the quarter-finals with a rollercoaster 6-2, 7-5 win over Spain's Carla Suarez Navarro.
Third seed Stephens, building up to the start of the defence of her maiden Grand Slam title in New York later this month, squandered three match points at 5-1 in the second set.
That setback allowed Suarez Navarro to battle back to 5-5 before the American steadied herself to claim a third win in three meetings against the Spaniard after 90 minutes on court.
Stephens hit 34 winners to the qualifier's 13 and goes on to face Anastasija Sevastova of Latvia who beat German 10th seed Julia Goerges 6-3, 7-6 (7/2).
Australian 15th seed Ashleigh Barty also made the quarter-finals by seeing off Alize Cornet of France 7-6 (7/3), 6-4.
Cornet had knocked out Wimbledon champion Angelique Kerber in the second round on Wednesday.
Barty next faces either Petra Kvitova, the Czech eighth seed, or Kiki Bertens of the Netherlands.
Britain's Johanna Konta saw off two-time Grand Slam champion and former world number one Victoria Azarenka, 6-3, 6-1, to move into the third round in a tie held over from Wednesday due to rain.
Konta will return to the court later Thursday to face fifth seed Elina Svitolina of Ukraine for a place in the last-eight.