Miami - 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.
Austrian 14th seed Dominic Thiem moved into the fourth-round path of world number one Novak Djokovic by dispatching 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.
Nishioka, a 20-year-old prodigy ranked 124th in the world, ousted Spanish 21st seed Feliciano Lopez in the second round for his first victory over a top-70 player.
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.
Britain's Jamie Murray, the older brother of two-time Grand Slam singles champion Andy Murray, could become world doubles number one later Sunday.
But Brazil's Marcelo Melo will keep the top spot if he and Croatian partner Ivan Dodig defeat Filipino Treat Huey and Belarussian Max Mirnyi to reach the quarter-finals.