They are two different players and, at times, it looks as if they are playing two different games.
Both players are through to the fourth round of the US Open and could collide in the semi-finals.
The 28th-seeded Williams is the only one of the eight women left in the draw who have won a grand slam title, which could answer the question of whose style works better in the sport's biggest events.
But Wozniacki is the top seed here and No 1 in the rankings - a system that rewards consistency more than big moments. By that standard, it is, indeed, hard to find anyone better. "I know I can be out there for hours and hours," she said.
"It's great to know that, and it's great to know that it doesn't matter how long the match takes. I mean, I will not lose because I'm not physically well."
Her stamina was clearly too much for Svetlana Kuznetsova in their third-round match.
Kuznetsova, the two-time major champion, had led 4-1 in the second set, but then lost 12 of the next 14 games.
"She was a wall," Kuznetsova said. "To break a wall, you cannot hit hard all the time. You have to mix it up. I think I did the right things. I just couldn't close it up in the right moments."
If Wozniacki grinds opponents down then Williams has the ability to blow her opponent away. Does she try to be intimidating on the court? "No, I don't try," she said. "I just am."
Roger Federer can be pretty intimidating, as well.
Federer has not missed a grand slam quarter-final since the 2004 French Open, and his quarter-final will be against No 11 seed, Jo-Wilfried Tsonga, who overcame a two-set deficit against Federer in the Wimbledon quarter-finals to win earlier this summer.
"[It] Doesn't necessarily need to be the next grand slam, but it's sure something I'm looking forward to," Federer said of the rematch. "I live for the big matches, live for playing a guy who is explosive, has got some firepower. I like to play those kind of players, especially now that we're in the deeper stages of the tournament."
Novak Djokovic's quarter-final will be against his Serbian Davis Cup teammate, Janko Tipsarevic, the No 20 seed, who defeated the 2003 French Open champion, Juan Carlos Ferrero.
The Serbs play together a lot, which might give Tipsarevic the advantage of some familiarity.
"But those things are maybe two per cent of the overall outcome of the match," Tipsarevic said. "You know, who plays better tennis is going to win. It's as simple as that."