Maria Sharapova scraped into the Kremlin Cup quarter-finals when Anna-Lena Groenefeld retired with an ankle injury when leading their second-round match 6-1, 4-2 on Wednesday.
The unseeded German was 4-1 ahead of the world number one and top seed in the second set when she slipped and twisted her left ankle.
After a medical time-out during which the ankle was taped, Groenefeld, on the brink of tears, played three more points before quitting.
"I knew I didn't win this match fairly. I don't like such victories," Sharapova told reporters.
"But strange things do happen in tennis and today's match was one of them."
Groenefeld had dominated Sharapova from the start.
The Florida-based Russian, who was making her Kremlin Cup debut, was broken three times in the opening set, losing her serve to love in the third and seventh games.
"I would be lying if I said that I was not nervous before the match," the 18-year-old said.
Sharapova again surrendered her serve in the fourth game of the second set when she hit a lame forehand into the net on break point to fall 1-3 behind.
Groenefeld won the next game and held two break points in the sixth before stumbling on the baseline as she tried to reach Sharapova's forehand.
"I knew she was a strong player and played well in Beijing but I must admit I just wasn't myself on the court today," said Sharapova, who beat Groenefeld 6-2, 6-2 in their only previous encounter at the German Open this year.
"It's always tough to come back after injury and I haven't played for two weeks, so obviously I need some time to find my rhythm. Hopefully, I'll play better in my next match."
Sharapova has not played since pulling out of her semi-final with a chest injury at last month's China Open, where Groenefeld reach the final before losing to Maria Kirilenko.
Sharapova began tentatively and had to save a break point in the opening game before going ahead 1-0. But it was all Groenefeld after that as she pinned the 2004 Wimbledon champion to the baseline.
A 10,000-strong home crowd, including Moscow Mayor Yuri Luzhkov, urged Sharapova on with chants 'Masha, Masha', as she tried to stay in the match.
"The crowd definitely helped me today. It was nice to hear them yell my name. The atmosphere was very much Russian," said Sharapova, who left her homeland 11 years ago to pursue her career in the United States.
