1. Nadal drops out at Aussie Open

Top seed withdraws in fifth set with injured right leg.

After an injury forced No. 1 seed Rafael Nadal to retire against Marin Cilic at the Australian Open on Tuesday, the Spaniard sounded an alarm on what’s sending so many men’s stars to the sideline.

MELBOURNE, AUSTRALIA — One point after his medical timeout in the fourth set, Rafael Nadal went to the rear of the court to squat and do a knee-raise, trying to stretch out his injured right leg.

Three games later — one point after his subsequent visit from the trainer — Nadal had to delay Marin Cilic’s serve while trying to walk out the pain at the start of the fifth set.

After limping and wincing through two more games, and after failing to fend off a sixth break point, the 16-time major champion and top seed was out of the Australian Open.

The sixth-seeded Cilic advanced to his first semifinal in Australia since 2010 with a 3-6, 6-3, 6-7 (5), 6-2, 2-0 victory Tuesday. He will play Kyle Edmund, who upset third-seeded Grigor Dimitrov 6-4, 3-6, 6-3, 6-4 to reach his first Grand Slam semifinal.

“Tough moments — not (for) the first time here,” Nadal said. “I’m a positive person, but today is an opportunity lost to be in a semifinal for a Grand Slam and fight for an important title for me. It’s really tough to accept.”

Injuries to star players dominated headlines before the tournament. Five-time Australian Open finalist Andy Murray withdrew so he could have surgery on his hip. Nadal (right knee), six-time champion Novak Djokovic (right elbow) and 2014 champion Stan Wawrinka (left knee) left their fitness decisions to the eve of the tournament, so it’s no surprise they are all out of the tournament.

There are limits to human endurance, and Nadal, 31, found his Tuesday.

“Somebody who is running the tour should think (a) little bit about what’s going on. Too many people getting injured,” Nadal said. “I don’t know if they have to think a little bit about the health of the players. I don’t know if we keep playing in this very, very hard surfaces what’s going to happen in the future with our lives?”

Elise Mertens upset fourth-seeded Elina Svitolina 6-4, 6-0 to extend her winning streak to 10 matches, becoming the first Belgian since Kim Clijsters in 2012 to reach the semifinals.

Up next is second-seeded Caroline Wozniacki, who finished off a 6-0, 6-7 (3), 6-2 win over Carla Suarez Navarro after 1:30 a.m.

Wozniacki, who is still chasing her first Grand Slam title, can also return to No. 1 for the first time in six years depending on results in Melbourne.

Second-seeded Roger Federer is still in contention for his 20th major, with a quarterfinal against Tomas Berdych today. On Federer’s side of the draw, Hyeon Chung and Tennys Sandgren are playing for a spot in the semifinals.

On Wednesday, 2016 champion Angelique Kerber advanced to the semifinals with a 6-1, 6-2 win over Madison Keys in 51 minutes.

Keys reached the U.S. Open final in September but lost in straight sets to Sloane Stephens.

Noteworthy: The USTA said Serena Williams will return to competition for the first time in more than a year, a little more than five months after giving birth, for the United States in its first-round Fed Cup match against the Netherlands in Asheville, N.C., on Feb. 10-11.

DITA ALANGKAR / AP Rafael Nadal receives treatment from a trainer during his quarterfinal loss to Marin Cilic on Tuesday. “It’s really tough to accept,” Nadal said of withdrawing.