"I wasn't missing that much", said the 26-year-old, who showed no sign of the foot injury that forced her to withdraw from the Qatar Total Open in Doha last month and appears poised to regain the title she won in 2015.
Halep advances to the quarter-finals and will face unseeded Petra Martic of Croatia, who advanced to the last eight with a straight sets 6-3, 7-6 (7-4) win against Czech Marketa Vondrousova on Tuesday.
Meanwhile, Venus Williams survived a hard fourth round test to beat Latvia's Sevastova 7-6 (8/6), 6-4 just one day after beating sister Serena in a third round contest.
She and Serena usually meet later in the draw in the semis or the championship match. "I knew that I had to change something, but I didn't realise what it was exactly", she said.
To reinforce their hopes, Venus has to beat Carla Suarez Navarro or wildcard Danielle Collins in the quarterfinals.
Simona Halep shared a picture of herself watching Serena and Venus on Monday night, writing: "Tonight I'm just a fan of the game and the Williams sisters".
Halep said she is happy with the way she is playing at this stage in the tournament. "That's why I'm trying just to go in to watch every time I can".
Monfils used an eight-minute injury timeout in the second set to get his back worked on before returning to the court and getting broken in fourth game.
Herbert moved on to a fourth-round matchup with Philipp Kohlschreiber of Germany, who upset No. 2 seed Marin Cilic 6-4, 6-4. Pliskova won the first seven points of the match and made her opponent commit 13 unforced errors to just 5 winners.
Herbert remains in contention for the $1 million bonus offered to a player who sweeps the singles and doubles titles.
With Cilic, Ferrer, Novak Djokovic, Monfils and John Isner all eliminated from his quarter of the draw, things are looking wide open for del Potro, the 2009 U.S. Open champion.
"I'm not thinking about that". "Still far away from the final, but of course, any chance to play with Roger would be great". Mayer next plays del Potro.
No.5 seed Karolina Pliskova ended the fairytale week of the ninth 16-year-old player to make R16 at Indian Well.The Czech outclassed 6-1 7-6 (2) Amanda Anisimova, the youngest player to record a Top 10 win since 2005 when Nicole Vaidisova defeated then No.9 Nadia Petrova to win the title in Bangkok.