They might be friends off the court, but No. 1 Serena Williams showed no mercy to No. 10 Caroline Wozniacki in the U.S. Open women’s final match Sunday. Williams clinched her 18th Grand Slam title by defeating Wozniacki in straight sets 6-3 6-3, tying greats Martina Navratilova and Chris Evert for fourth on the all-time slams list, while capturing her third straight U.S. Open and sixth overall.

It was also the third time in Williams’ career that she won it all at New York’s Flushing Meadows without dropping a single set. Wozniacki could manage only four winners while Williams had 29 during the match that lasted 1 hour 15 minutes.

Wozniacki, who spent 67 weeks at No. 1 from 2010 to 2011, is still seeking the first major title of her career. It was the second appearance in the U.S. Open final for the Danish star, who fell to Belguim’s Kim Clijsters in straight sets back in 2009.

Although Williams was eliminated in the early rounds of the French, British and Australian Grand Slam events in 2014, she was determined to end the year on a high note. She also claimed a record $4 million for the victory. Including the $1 million prize for winning the Emirates Airline U.S. Open Series just before the event, and the $3 million overall purse, Williams will take home the most lucrative prize for a single tournament ever for a man or woman.

Halfway through the first set, Williams lost several break points because of her 16 unforced errors, allowing Wozniacki to stay alive early despite having only one winner at that point.

Williams’ errors allowed Wozniacki to force a ninth game in the first set, but the 32-year-old then overpowered her opponent with her serve with three aces, going 8-for-10 on her net approaches.

Williams tightened up her game in the second set and started to roll. She forced Wozniacki from baseline to baseline, and committed only three errors in the take a 2-1 lead.  Yet, despite Williams’ dominance, Wozniacki battled back in the second set to make 4-3.

But Williams continued to tire Wozniacki, with a 26-shot rally leading to a Wozniacki error and a double match point.