Here’s what happened on Day 3 of the U.S. Open:

Muguruza Takes a Late-Night Loss

Garbiñe Muguruza’s struggles continued with a 3-6, 6-4, 6-4 loss to the qualifier Karolina Muchova, which ended after 1 a.m. The 12th-seeded Muguruza has won only three matches since reaching the semifinals of the French Open. As the defending champion at Wimbledon, she lost in the second round. She also withdrew from two hardcourt tournaments with an arm injury.

Muchova, 22, is ranked 202nd and is playing in the main draw of a Grand Slam event for the first time.

Confusion About the Heat Policy

With the temperature in the mid-90s for the second day in a row, the men again played under a new extreme heat policy, which calls for a 10-minute break between the third and fourth sets.

On Tuesday, the first day the policy was used, five men retired from matches with heat-related problems. But on Wednesday, the only man who retired from a match, Jason Kubler, did so because of an ankle injury.

The policy has created relief — and confusion. It allows for players to “go anywhere” for a respite from the heat, but prohibits coaching and off-court medical treatment. Andy Murray said he saw at least one violation of that protocol during his second-round loss to Fernando Verdasco on Wednesday.