The 35-year-old small forward posted a game-high 43 points in a rare 45 minutes of play.
"She started really well right away, and had really good length, and was serving great. So I struggled at the beginning. I felt like I'm also not playing too bad, but just so many easy points going away with serve and return situations," said Siegemund.
But Nadal was better in Grand Slam clashes (9-6), especially on his favourite clay court (6-1).
"When I started my career I didn't imagine or dream of being part of a fourth qualifying tournament. For me it's a source of pride. I'm enjoying it to the fullest, in the knowledge that my career is drawing closer to an end."
"Definitely eight years ago I was young, I was already won my Wimbledon title, and I was kind of favorite and played and I lost to Maria, which she won the tournament. So for me, that time was a lot of pressure on me, and even I didn't play like the best on clay," the two-time Grand Slam winner added.
"I also had a lot of days in my recovery when I wondered how I was going to play football again, but there were also good days that I'll never forget, such as starting to walk and it not hurting and then being able to run and in my first game back I scored after just 25 seconds."
Messi put the hosts ahead in the 13th minute at La Bombonera after Lucas Ocampos was scythed down by Pervis Estupinan after a driving run into the penalty area. The 33-year-old now has 71 goals in 139 appearances for the Albiceleste.
Now the top two seeds - Djokovic and Nadal - will face each other, which would offer big stakes for both sides to seek for.
Shao, captain of China's national team, first stood out to convert two fast break baskets in a row. Sichuan showed its depth with substitutes on the court, compiling a 10-point advantage after the first quarter expired.
In the quarterfinals, SN is to play JDG, while TES will meet European second seed FNC.
The teenager will see her world ranking improve to the No. 17 after this greatest win of her career up to date. Enditem