Andy Murray eases past Feliciano López to keep up Sony Open defence

Andy Murray en route to a 6-4, 6-1 win against Feliciano López of Spain in the third round of the Sony Open in Miami. Photograph: Clive Brunskill/Getty Images

Andy Murray, the defending champion, eased into the fourth round of the Sony Open in Miami with a straight-sets win over Spain's Feliciano López.

Murray extended his perfect record against López to 9-0 with a 6-4, 6-1 win in 73 minutes to set up a tie with France's Jo-Wilfried Tsonga, who recovered from 5-1 down in a second-set tie-break to beat Marcos Baghdatis in three sets.

The Scot made the ideal start with a break of serve to love in the opening game of the match but there was a moment of concern for Murray in the next game when he grabbed his lower back in pain after dumping a forehand into the net.

Murray went on to lose that game after saving two break points but did not appear to be physically hampered afterwards and he soon broke López again for a 3-2 lead. He lost only one point in each of his next three service games to serve out the set and again broke López at the start of the second set.

The Spaniard looked to be suffering from back problems of his own and never threatened to get back into the contest, losing his serve twice more as Murray sealed victory with a casual backhand return winner down the line.

"I moved well [and] returned well," Murray said. "It's not always that easy to feel comfortable against him because there is not [a lot] of rhythm with the way he plays."

Asked afterwards about the split with his coach Ivan Lendl, Murray joked: "We're back together again. It was only a four or five-day split. No, it's a shame he won't be watching many more of my matches from the stands.

"I'm just looking forward to try and do more in the big events."

Tsonga, the 11th seed, had to rally for a 4-6, 7-6, 7-5 win over the Cypriot wildcard Baghdatis.

Meanwhile Roger Federer secured another comfortable win, beating qualifier Thiemo de Bakker in straight sets.

The fifth seed lost only seven points on his serve as he ran out a 6-3, 6-3 winner in 63 minutes, setting up a fourth-round match with Frenchman Richard Gasquet.

"We have played many times and I'm not sure how he's playing right now," Federer said of the Frenchman. "I have to find a little bit. He's got a great game and gives himself time, sort of that extra second of time on each shot.

"He's a good all around player. With the wind and the slower conditions, it works quite well for him. It's going to be tough match for me."

Earlier Li Na overcame three set points to beat the American Madison Keys 7-6, 6-3 in the women's draw. Li, the No2 seed and reigning Australian Open champion, won even though she served seven double-faults and was broken four times.

Keys, serving at 5-3 in the first set, failed to convert her set points. She also squandered a lead in the second set, when three times she was within a point of going 3-0 up against Li.

"She's No2 in the world for a reason," Keys said. "She just won the Australian Open for a reason. She's a great player. I played well at times, and she just played the bigger points really well."

The American Coco Vandeweghe earned a shot at Serena Williams in Monday's fourth round by beating 2011 US Open champion Samantha Stosur 5-7, 7-5, 7-5.

The match began shortly before midnight on Saturday and ended with a handful of spectators in the stadium.