OSAKA, Japan - No. 4 seed Angelique Kerber became the first player through to the semifinals of the Toray Pan Pacific Open on Friday as No. 5 seed Madison Keys retired due to injury early in the deciding set of the opening match on Center Court.

After the pair split two tight sets and with the match clock at just over two hours, the German broke to love to lead in the final set at 6-4, 4-6, 2-1, before Keys could no longer continue.

"It was a really good match in the first two sets, and this is never the way that you want to win the match," Kerber said. "I hope Madison will recover really soon so she is ready for the next tournaments.

"I tried to give everything I could today, and I'm looking forward to playing the next round."

The abridged victory is Kerber's second straight retirement win over Keys, dating back to last year's Dongfeng Motor Wuhan Open, and improves her head-to-head against the American No. 3 to 9-2. 

In what was a high-quality match prior to Keys' retirement, both players recorded more winners than unforced errors. Keys racked up 39 winners to 33 unforced over the course of the match, with Kerber's expert counterpunching leading to 18 winners and 13 unforced. 

During the match, each player rallied from a break down in the set that she won. In the opener, it was Keys who started the stronger of the two as she won the first two games before Kerber broke back to get on the board.

The German scored the deciding break of serve in the seventh game, which was sandwiched around a pair of service holds in which she was forced to save break point to keep Keys at bay.

In the second set, Kerber was the first to lead as the two players traded a pair of early breaks, but it was the World No. 16 who ultimately came out the victor despite a pair of medical timeouts and a brief rain delay. 

After saving a break point to keep her lead on serve in the seventh game, Keys ultimately forced a final set with her third break of the set.

With the victory, Kerber advances to her first semifinal since she finished runner-up at the Nature Valley International in Eastbourne in June, and seals her fifth semifinal showing in six appearances in Tokyo since 2012.

"I think I like to play in Japan. Always when I'm coming back, I feel good here and I'm playing good tennis," the three-time Grand Slam champion said.

"This year, I'm here to play as good as I can and I'm looking forward to playing another semifinal here."

Inclement weather conditions ensured this would be the only match completed on Friday, with Anastasia Pavlyuchenkova 2-0 up in the first set on Misaki Doi when the rain started to fall.