* Djokovic reaches quarters after having played two matches
* Ferrer falls short in bid for second straight Miami final
* Dolgopolov upsets Australian Open champion Wawrinka (Adds quotes, later matches)
By Steve Keating
MIAMI, March 25 (Reuters) - Defending champion Andy Murray swept past Jo-Wilfried Tsonga at the Sony Open on Tuesday to set up a tantalising quarter-final showdown with three-time Miami winner Novak Djokovic.
Murray, playing his first event since splitting with coach Ivan Lendl last week, has looked increasingly comfortable and confident on his own, disposing of the 11th-seeded Frenchman 6-4 6-1 in a tidy 73 minutes.
Djokovic, riding the momentum from his Masters series win in Indian Wells nine days ago, also continued to display superb form in taming Spaniard Tommy Robredo 6-3 7-5.
Joining Murray and Djokovic in the last eight are Japan's Kei Nishikori and Ukraine's Alexandr Dolgopolov after each registered a pair of fourth-round upsets.
Dolgopolov gave Ukrainians a little something to discuss beside political turmoil by battling to a 6-4 3-6 6-1 win over third seed and Australian Open champion Stanislas Wawrinka while Nishikori shocked fourth seeded Spaniard David Ferrer 7-6 (9-7) 2-6 7-6 (11-9).
"I need to continue to play well and bring those positives to the country," said Dolgopolov, who was born and raised in Kiev. "Politically there is nothing I can do, I'm not there I just live with it and hope for the best."
While the day got off to a gloomy start with early morning showers delaying the start of play by 30 minutes, tennis fans were treated to a lineup of sparkling action with nine of the world's top 10 men battling for spots in the quarters.
Djokovic had only two break points the entire match and that was all the second seeded Serb needed converting both chances.
It was just the second match of the tournament for Djokovic, who received an opening-round bye and advanced to the last 16 on a walkover when Florian Mayer withdrew with a groin injury,
"Very solid win," said Djokovic. "I just was telling myself to stay mentally tough out there and composed and not get carried away by few points.
"I have done well. I have served well. I made him play an extra shot in important moments and that's why I'm satisfied with the overall match today."
Murray was no less efficient never facing a break point.
The Scotsman registered the only break of a tight first set but then, after allowing the Frenchman to hold serve to open the second, swept the next six games to complete a clinical victory.
Nishikori, who had never been past the fourth round in Miami, entered new territory after outlasting Ferrer, one of the fittest players on the ATP Tour and a finalist here last year.
The roller-coaster contest began with a first set that featured three breaks of serve by each player before Nishikori won it in a tiebreak.
Ferrer hit back by breaking the 20th-seeded Japanese to open the second and again to go up 4-1 on way to leveling the match.
The up-and-down battle continued into the third with the two men exchanging a pair of early breaks before holding serve to the finish, which ended in another gripping tiebreak and Nishikori again coming out on top.
"I was ready for longer rallies, I didn't think it was going to be this long a match but to beat him in three sets that shows me confidence I have," said Nishikori. "To go to quarter-finals here the first time is what I want to do at these Masters and big tournaments and hopefully I can keep going." (Editing by Frank Pingue)