* Williams blitzes Vandeweghe in routine 79-minute match
* Sharapova forced to overcome sluggish start to advance (Adds Williams quotes, results)
MIAMI, March 24 (Reuters) - Six-times Miami champion Serena Williams and five-times runner-up Maria Sharapova remained on another Sony Open collision course after each recorded fourth-round wins on Monday.
While Williams and Sharapova clashed in last year's final there will be no championship rematch on Saturday at Crandon Park with the American and Russian on the same side of the draw and closing in on a semi-final showdown.
After getting her title defence off to a slow start world number one Williams, who lives an hour's drive from the Crandon Park Tennis Center and considers the event her home tournament, stepped it up a gear against Coco Vandeweghe sweeping past the young American qualifier 6-3 6-1 in 79 minutes.
"I was definitely happier today," Williams told reporters. "I was really struggling my first two matches, so I just wanted to have a better performance today.
"Going into the match I knew I could only do better. That kind of helped out, too."
Fourth seed Sharapova got a much-needed wake-up call after sleep-walking her way through the opening set before dispatching Belgian Kirsten Flipkens 3-6 6-4 6-1.
Former world number one and 12th seed Ana Ivanovic seemed headed for a quarter-final berth after comfortably taking the first set against eighth seed Czech Petra Kvitova but fell apart after that, committing 11 double faults en route to a stunning 3-6 6-0 6-0 loss.
Williams, a minority owner of the National Football League's Miami Dolphins, once again stepped onto the court dressed in her team's turquoise and orange colours and kicked off the match by claiming the only break of the opening set for the early lead.
In the second set, Williams simply blitzed Vandeweghe, overpowering the 22-year-old who managed to hold her serve just once against the 17-times grand slam winner.
"It doesn't feel great (playing bad tennis)," said Williams. "That also gave me confidence to know if I'm winning these matches when I'm playing some of the worst tennis I have personally played in the past couple of years, then, you know, it gave me a lot of hope.
"I'm in a better mood now. It was impossible for me to be in a good mood after I played those last matches."
Sharapova, playing the first match of the day on a drowsy and overcast centre court, appeared to have trouble getting up for her fourth-round contest, as did many of the ticket holders with only a few hundred spectators sprinkled across the quiet stadium as play began.
"I started off ... probably looked like it was too early," Sharapova told reporters. "I usually like playing first match on, but I didn't start off the way I wanted to.
"Nothing was working. Just the way it went in the beginning."
Flipkens certainly came ready to play and broke a misfiring Sharapova at the first opportunity and again to go up 4-0 with the help of back-to-back double faults from the Russian.
In the second set, a suddenly alert and focused Sharapova turned the tables on the 19th seeded Belgian as she raced in front 4-0 on a pair of breaks on way to leveling the match.
Trailing 3-1 in the third set, Flipkens had a glorious opportunity to get back into the contest after going up 0-40 on Sharapova's serve but was unable to convert on any of her four break chances.
Sharapova weathered the storm then closed out the match by sweeping the next two games to reach the last eight.
"You want to be able to hold the winner's trophy, but you also know the matches that you got through to get in the position to get to the final stage," said Sharapova. "I had my opportunities.
"It's not like I didn't have my opportunities in those finals. I just didn't take them." (Editing by Frank Pingue)