The 30-year-old earned the award for the first time in her career in the qualifiers category. 

She triumphed over 23-time Grand Slam champion Williams in Everett in Washington in February, ending the American's unbeaten singles streak in the team competition that dated back to 1999.

"I'm very honoured to be included in the Fed Cup Heart Award this year," said Sevastova.

"It's a great achievement and privilege."

Sevastova is set to donate her $3,000 (£2,500/€2,800) prize money to Latvian food bank charity Paedusai Latvijai. 

She was one of four winners chosen in a public vote, with the remaining three awarded a prize of $2,000 (£1,600/€1,900). 

Sania Mirza of India received the Asia and Oceania award, having played an integral part in her country's play-off success to reach the Fed Cup. 

"It's an honour to win the Fed Cup Heart Award as the first Indian," she said. 

"I dedicate this award to the entire country and to all my fans and thank everyone for voting for me. 

"I hope to bring more laurels to the country in the future."

Fernanda Contreras Gomez was the winner for the Americas category, having emerged victorious from all but one of her singles and doubles matches as Mexico reached the play-offs. 

"I'm very grateful," Contreras Gomez said. 

"I couldn't have done it without my team, my friends, my family and my country. 

"I have no words to describe how I feel but the most important thing is that we're going to help the children who need the most love and help."

Anett Kontaveit of Estonia earned the accolade for Europe and Africa. 

The world number 22 extended her unbeaten singles run in the competition to seven matches, taking her country to the brink of qualification for the Fed Cup.

"I am really grateful to everyone who took their time to vote for me," Kontaveit said. 

"Representing my country has always been important and special to me."

The Fed Cup Heart Award is an International Tennis Federation (ITF) initiative that was first presented at the Final in 2009. 

It aims to recognise players who have represented their country with distinction, shown exceptional courage on court and demonstrated outstanding commitment to their team. 

"Fans are at the heart of Fed Cup so to be chosen by the fans as a winner of an award that recognises the competition's values of team work, courage and national pride is extra special for the players," said ITF President David Haggerty.

"These values, and all the players nominated for the Fed Cup Heart Award, are what make Fed Cup such a unique and special competition."