From 2-1 down in set number two, Serena won five games in a row to bring the match home in style, earning a break at 2-2 and another one in game seven with a forehand victor before closing the match with a good serve in the following game.
In the earlier men's singles match, No.15 ranked Tsitsipas overcame a second set wobble to defeat Tiafoe 6-3, 6-7 (3), 6-3. It means Federer and Williams will play against each other in the mixed doubles portion of that match. "I have always thought how is it to return that serve and go head-to-head with her".
The fancied Greece kept its tournament alive with a 4-1, 1-4, 4-2 victory in the mixed doubles after an opening defeat against Britain. "I hope tennis fans tune in and watch it because it's going to be one time and probably never again in this type of competition", Federer said.
The 23-time Grand Slam champion reported some soreness in her ankle, but hit 13 aces and 29 winners en route to victory.
Despite their concurrent careers, Federer said it would be an "exaggeration" to say he knew Williams well.
Tsitsipas repelled them all with winners and the match was in his hands after a great backhand cross court victor at 5-3 that pushed Greece in front before the women's singles match.
"This time it's against Serena, who is one of the biggest champions ever in our sport - men and women combined".
EPL results, scores | Liverpool thrash Arsenal 5-1, Wolves beat Tottenham
Liverpool are nine points clear of second-placed Tottenham , who were stunned 3-1 at home by Wolverhampton Wanderers. During those game weeks, Liverpool have defeated Newcastle 4-0, Wolves 2-0, Bournemouth 4-0 and Brighton 1-0.
Roger also had the opportunity to bagel (6-0) his opponent in the first set when he had 2 set points when leading 5-0, but the Brit managed to hold his serve.
Bencic and Federer are the defending champions.
"I don't know her that well", he said.
Williams said it was "like a dream come true".
Federer and Bencic will next take on Williams and Frances Tiafoe to kick-start the New Year with arguably the most anticipated match in the tournament's three-decade history.
"It was very good".
"I wasn't missing my rhythm and it was flawless conditions, and I was happy with all aspects of my game".