MIAMI (AFP) - Reigning Australian Open champion Angelique Kerber rallied to reach the fourth round of the ATP and WTA Miami Open on Sunday when 108th-ranked Dutch qualifier Kiki Bertens retired due to illness.

The German second seed, who upset top-ranked Serena Williams two months ago at Melbourne for her first Grand Slam title, was leading 1-6, 6-2, 3-0 when Bertens met her at the net and said she could not continue.

"I was not finding a rhythm at the beginning of the match," Kerber said. "I had to stay strong, believe and keep fighting. I hope she gets well soon but I'm glad to be in the next round."

Kerber next faces Hungary's 49th-ranked Timea Babos, who ousted 18-year-old Japanese 104th-ranked wildcard Naomi Osaka 7-5, 6-0. Kerber is 3-0 against Babos, with 2012 wins at Cincinnati and the London Olympics and another in last year's first round of the French Open.

The 28-year-old left-hander became the first German woman to win a Slam crown since Steffi Graf in 1999 at Wimbledon, but since then Kerber had lost opening matches at Doha and Indian Wells after withdrawing from Doha with a right thigh injury.

After exchanging breaks to open, Bertens broke Kerber twice more - when the German swatted a forehand wide and on her own backhand drop volley winner - then held to claim the first set in 31 minutes.

Kerber held to 3-2 in the second set when Bertens called for a trainer to help combat the heat and stomach pain. Bertens fought on but Kerber won the next six games before her rival retired.

Eighth-ranked Victoria Azarenka, who won 2009 and 2011 Miami titles, eliminated Polish qualifier Magda Linette 6-3, 6-0.

The 26-year-old from Belarus is trying to become only the third woman to sweep the Indian Wells-Miami US double after Graf in 1994 and 1996 and Kim Clijsters in 2005.

Azarenka hit a backhand winner to break for the first set, then saved two break points in an 11-minute opening game of the second set and breezed to victory from there.

"I won the first set and I took the energy and I stayed on top," Azarenka said.

Argentina's Horacio Zeballos, a lucky loser who replaced Roger Federer in the draw when the 17-time Grand Slam champion withdrew due to illness, advanced to the fourth round, rallying to beat Spain's Fernando Verdasco 1-6, 6-4, 7-6 (7/4).

Zeballos, ranked 112th, had not won a match since last July before beating countryman Juan Martin Del Potro in the second round. He will face Belgian 15th seed David Goffin on Tuesday for a quarter-final berth.

Czech seventh seed Tomas Berdych reached the last 16, outlasting US 31st seed Steve Johnson 6-3, 6-7 (6/8), 6-3.

Austrian 14th seed Dominic Thiem moved into the fourth-round path of world number one Novak Djokovic by dispatching 124th-ranked Japanese qualifier Yoshihito Nishioka 6-2, 6-2 in 64 minutes.

Thiem captured his fifth career ATP title last month by beating Australia's Bernard Tomic in the Acapulco final.

Two-time defending champion Djokovic, who won his 11th Grand Slam crown at the Australian Open two months ago, first must get past Portugal's 38th-ranked Joao Sousa in a later match at the hardcourt event.

Djokovic, trying to match Andre Agassi's record of six career Miami crowns, has won 11 Miami matches in a row and 25 of his past 26 matches at the venue, the only loss in the span coming to Tommy Haas in 2013.

The 28-year-old Serbian also has won titles at Doha and Indian Wells this year and hopes to complete the US double sweep at Miami and Indian Wells for the third consecutive year.