Serena Williams of the United States reacts in her semifinal match against Madison Keys of the United States during day 11 of the 2015 Australian Open at Melbourne Park on January 29, 2015 in Melbourne, Australia

Serena Williams of the United States reacts in her semifinal match against Madison Keys of the United States during day 11 of the 2015 Australian Open at Melbourne Park on January 29, 2015 in Melbourne, Australia

*Serena Williams dug deep to win a tough first set against 19-year-old Madison Keys, then proceeded to dominate the second set in a 7-6 (5), 6-2 victory to move into an Australian Open final against second-seeded Maria Sharapova.

Williams, an 18-time Grand Slam champion, takes a 15-match winning streak against Sharapova into the women’s final. It will be the first time a No. 1 plays a No. 2 at the annual tournament since 2004.

The 33-year-old Williams will be the oldest player to reach the Australian Open final in the Open era. She won each of the previous five Aussie Open title matches she played in.

Sharapova won the 2008 title, but was comprehensively outplayed in her two other trips to the final — by Williams in 2007 and by Victoria Azarenka in 2012.

Williams, who has been struggling with a cold for a week, said she’d recover from a tough workout in the all-American semifinal against Keys, who pounded her with heavy groundstrokes and a big serve for the first set.

“She pushed me really hard the first set … and I had to really dig deep mentally to get through that,” Williams said. “It was a little frustrating. I had like nine or 10 match points and couldn’t close it out. That doesn’t happen so much. She played like she didn’t have anything to lose.”