Martina Hingis has been warned for "coaching" after her mother was caught shouting instructions to her, during a match against Belgium's Kim Clijsters.
The world number one was struggling through her second round match against Clijsters at the Sydney International, when her mother began telling her to use more top-spin from the sideline.
Hingis admitted her mother had shouted at her during the match, which is the lead-up event for the Australian Open but said she was only trying to encourage her, rather than coach.
"I got a warning for coaching, but I didn't even look at my mother," Hingis said after the match.
Hingis was also warned for coaching during last year's US Open semi-final, which she lost against Venus Williams.
Hingis had more luck against Clijsters on Wednesday but had to work hard for her 6-4,6-4 victories. The Swiss star said it had been good preparation for her match on Thursday against Serena Williams.
"I didn't play my best, but I was lucky it was good enough," Hingis said.
On Tuesday, Hingis and Monica Seles beat Venus and Serena Williams in the first round doubles match. It was the first time Hingis and Seles had teamed up.
In the day's other tennis action involving Swiss players, Roger Federer beat compatriot, Marc Rosset, 6-1, 6-2 in their second round match. Federer will now take on Sebastien Grosjean of France.
In Canberra, Switzerland's Patty Schynder lost in the second round. The sixth seed was defeated 6-0, 3-6, 7-5 by Japan's Ai Sugiyama.
swissinfo with agencies