Serena Williams hopes the back injury that saw her retire during the Rogers Cup final disappears after a "small window".

The American great was 3-1 down to Canadian teenager Bianca Andreescu in Sunday's decider in Toronto when she retired due to back spasms.

Williams, 37, was in tears on court, but revealed the spasms usually lasted less than two days.

"That's the most frustrating part. I've had this before and it's like 24-36 hours where I'm just in crazy spasm and then it's like gone," the 23-time grand slam champion told a news conference.

"That's a little frustrating for me because I know that I can play, I just can't play today.

"So, I don't know, I'm just taking it a day at a time and see, usually what happens is it continues like that with just that small window and see if I improve after that small window."

Williams enjoyed a fine run in Toronto, improving her win-loss record this year to 19-5.

The veteran said she initially felt her back during a tough three-set win over Czech qualifier Marie Bouzkova in the semi-finals on Saturday.

"It started yesterday in my match and it just got worse," Williams said.

"My whole back just completely spasmed and to a point where I couldn't sleep and I couldn't really move and so I was just trying to figure out how do you play a match where you have no rotation?"