Britain's Sears, who is also the father of Andy Murray's heavily pregnant wife, Kim, needed emergency treatment in the stands early in the second set, as an extremely worried-looking Ivanovic looked on.

After the match finally resumed, with both players showing great courage and composure, America's Keys fashioned a 4-6, 6-4, 6-4 come-from-behind win to reach the fourth round.

Sears, 58, was stretchered off and taken to hospital, but the stadium announcer later told the crowd he was conscious and sitting up. "He's okay," he said.

Key's win sets the 15th seed up with a crack at the quarter-finals if she can get past either Chinese giant-killer Zhang Shuai, who knocked out world number two Simona Halep, or America's Varvara Lepchenko.

Medics were called for Sears during the changeover when Ivanovic was leading 6-4, 1-0.

The Serb appeared extremely concerned as she looked on, before the players were taken off court as doctors attended him on a stairway. He was later stretchered away.

The Australian Open live blog identified him as Sears, who is not only Ivanovic's coach but the father of Murray's pregnant wife Kim Sears, who is due to give birth within weeks.

Murray was playing on the neighbouring Margaret Court Arena, watched by his mother Judy, apparently unaware of what happened.

A statement from tournament organisers later did not identify Sears.

"A male in his 50s fell ill in Rod Laver Arena this evening. He was transported to hospital by ambulance," it said.

"The match was suspended and the players updated on his condition. The players had the opportunity to consult with their teams and then went back on court."  

ESPN's courtside reporter, former player Pam Shriver, said she had been told by Ivanovic's team that Sears had complained of feeling unwell 30 minutes before the match.

She said he was conscious and had been taken to hospital.

Play resumed just under an hour after the incident with Ivanovic, who reunited with Sears last year after working with her between 2011-13, returning composed.

It was her second match in a row interrupted by a medical emergency. She was left badly shaken when an elderly spectator fell on a flight of stairs during her clash in the same arena on Thursday.

Despite the mid-match drama, she immediately broke Keys, a semi-final in Melbourne last year, and looked headed for victory, but the momentum kept shifting in an intense few games with both players breaking each other's serve.

Keys grabbed the advantage in a pulsating 10th game in which the Serb had six break points to stay in the set but could not convert any of them.

The American eventually levelled the match at one set apiece on her third set point.

Drawing on her experience, the fired-up 2008 French Open winner got a crucial break to go 2-1 clear. But the see-sawing match had more twists with Keys breaking back in the fourth game and again in the seventh.

She completed the come-from-behind win with a cross-court volley on her second match point as both players departed without doing courtside interviews.