Ana Ivanovic made the second round of the Qatar Open on Monday, but she was denied a full match as Carla Suarez Navarro retired with a back problem in the second set.
Ivanovic, the former world No 1, who has now slipped to 19th in the rankings, had won the first set 6-1 and was leading 4-1 in the second when her Spanish rival quit after receiving courtside treatment.
And the Serbian knows she has a long way to go before she reaches the level that saw her win the French Open in 2008.
"I think it's different in the sense that now a lot more girls can challenge top players from the early rounds," Ivanovic said.
"That's exciting for us, because we do want challenges.
"But I feel like those girls that are on top at the moment, they're playing very aggressively, and that's the way you have to play in order to win.
"It's a big challenge for us to raise our level."
But despite the challenges, the Serbian said her goal was to get as high in the rankings as possible.
"Well, that's why I'm here," she said when asked if she was targeting the number one status again. "If I didn't think so, I probably wouldn't be sitting here. But, yeah, I definitely believe so."
As the top eight seeds who have been given first round byes enjoyed a rest, the first upset of the event came when Kazakhstan's Ksenia Pervak defeated the 10th seed Anastasia Pavlyuchenko of Russia 1-6, 6-2, 6-3.
Israeli Shahar Peer and China's Shuai Peng also advanced, beating Arab wild cards Nadia Lalami of Morocco and Fatma Al Nabhani of Oman respectively.
Caroline Wozniacki is looking forward to playing in Doha without the burden of being called the world's top-ranked player. The Dane, the world No 4, began 2012 at top of the rankings, which she said had put her under pressure.
The winner of 18 titles said she is happy she does not have to fend off unpleasant questions about her not having a grand slam title to her credit.
"It's just nice that actually I don't get the question every press conference, 'You're No 1 but haven't won a grand slam'," she said.
"So at least I get that off my back and I can actually just play and people don't get disappointed if I lose a finals or semi-finals, which, you know, would be a disaster before.
"Now someone else can have that, and I can just play."