MELBOURNE — A quick look at the Australian Open:

LOOKAHEAD TO WEDNESDAY

Rafael Nadal's sore back seems to be improving as he heads into the Australian Open quarterfinals with an eye on a men's-record 21st Grand Slam singles title. The Spanish left-hander's opponent will be Stefanos Tsitsipas, who did not need to hit a shot in the fourth round because No. 9 Matteo Berrettini withdrew with an abdominal injury. Nadal is 6-1 against the 22-year-old Tsitsipas, including a 6-2, 6-4, 6-0 win in the 2019 semifinals at Melbourne Park. The other quarterfinal on their half of the draw will be No. 4 Daniil Medvedev, the 2019 U.S. Open runner-up, against No. 7 Andrey Rublev in an all-Russian matchup. Medvedev, 25, and Rublev, 23, have been friends since childhood. There's also a one-nation women's quarterfinal between Americans Jennifer Brady and Jessica Pegula. They are good friends off the court and have played doubles together. Pegula beat Brady in their only previous singles match, on a hard court ahead of last year's U.S. Open. Brady has not dropped a set through four matches at the Australian Open so far. Pegula beat fifth-seeded Elina Svitolina in the round of 16. The other women's quarterfinal has top-ranked Ash Barty of Australia playing Karolina Muchova of the Czech Republic. Barty beat Muchova at the 2018 U.S. Open in their only previous meeting. Barty is bidding to reach her second successive Australian Open semifinal and win her home Grand Slam for the first time. Both players are unbeaten in 2021 and have not dropped a set this tournament. 

WEDNESDAYS FORECAST

Sunny. High of 31 degrees Celsius (88 degrees Fahrenheit)

TUESDAY'S WEATHER

Mostly sunny, high of 31 degrees C (88 F).

TUESDAY'S KEY RESULTS (DAY SESSION) 

Women's Quarterfinals: No. 10 Serena Williams beat No. 2 Simona Halep 6-3, 6-3; No. 3 Naomi Osaka beat Hsieh Su-wei 6-2, 6-2.

Men's Quarterfinals: Aslan Karatsev beat No. 18 Grigor Dimitrov 2-6, 6-4, 6-1, 6-2. 

STATS OF THE DAY

Zero — Number of men in the Open era who had reached the semifinals of a Grand Slam debut until 114th-ranked qualifier Karatsev's surprising run in Australia.

19 — Consecutive matches won by Osaka, including last year's title at the U.S. Open.

QUOTE OF THE DAY

"She's Serena. I feel really intimidated when I see her on the other side of the court." — Osaka, on the possibility of facing Serena Williams in the semifinals.