Juan Martin del Potro sent his devoted fans home happy Tuesday by powering into the ATP Miami Open quarter-finals with a 6-4, 6-2 triumph over Serbian Filip Krajinovic.
Marin Cilic qualified for the quarterfinals in Miami with his 7-5 and 7-6 (4) victory over Canada's Vasek Pospisil.
Juan Martin del Potro and Marin Cilic are the biggest names left in a field lacking star power.
World No. 8 Del Potro extended his win streak over the Spaniard to five straight matches, winning 6-4, 7-6 (7/3) at the ATP Tour hardcourt tournament here Tuesday.
Roger Federer remained at the number one spot with 10,060 points.
Marin Cilic will play the winner of the first-round match between France's Gael Monfils and Argentina's Horacio Zeballos in the second round.
Roger Federer had previously won the Australian Open in 2004, 2006, 2007, 2010 and 2017.
Cilic will face either defending champion Roger Federer or unseeded South Korean Chung Hyeon, who play on Friday.
World number one Rafael Nadal faces three weeks out after being diagnosed with a torn inner hip muscle during his Australian Open quarter-final defeat
Cilic won 3-6, 6-3, 6-7 (5/7), 6-2, 2-0 in 3hr 47min on Rod Laver Arena and will face Britain's Kyle Edmund for a place in the final on Thursday.
The 33-year-old Frenchman did not disappoint his home crowd as he beat the Croatian in three sets 1-6, 6-3, 6-2 to reach the final of the tournament.
Top seed Marin Cilic from Croatia outplayed Pierre-Hugues Herbert while Kevin Anderson survived a mighty scare from Mikhail Kukushkin.
Jack Sock is the first American to reach the semi-finals of the ATP World Tour Finals event since Andy Roddick in 2007.
By extending his current winning streak to 13 matches, the 36-year-old advanced from the Tour Finals group stage with an unblemished 3-0 record for the 10th time in his glittering career.
Jack Sock, the American number one, came from nowhere to reach the ATP World Tour Finals, reaching London by winning the Paris Masters.
Rafael Nadal is chasing a seventh title in a season and a hat-trick of triumphs on the trot, having been crowned US Open and China Open champion in recent weeks.
With four matches a day -- three singles and a doubles, the players can earn a point for each win on Friday, two on Saturday and three on Sunday.
Former world number one Sharapova, the 2006 US Open winner in her first Grand Slam since serving a 15-month doping ban, downed the 139th-ranked wildcard 7-5, 6-2 and moved into a fourth-round clash with Latvian 16th seed Anastasija Sevastova.