The 26-year-old is the first Romanian to hold the ranking.
Simona Halep was a victory away from becoming world number 1 3 times this season and she couldn't stand the pressure, losing all those matches, but that all changed here in Beijing.
The second set started with 4 good holds, which was important for Ostapenko to regain her confidence, but she got broken in game 5 after netting an easy forehand, which was her 25th unforced error since the start of the match! "Many others did it, and I'm one of them", said Halep following her win. Along the way, Halep will become the co-author of a repetition of the record for the second time in history during the season on top of the world rankings will be five players (Angelique Kerber, Serena Williams, Karolina Pliskova, Garbine Muguruza and Simona Halep).
Simona Halep has become the fifth World No 1 player on the women's tour after she progressed into the final of the China Open in Beijing.
But the Romanian is in red-hot form and claimed the scalp of former world No. 1 Maria Sharapova earlier this week, beating the Russian at the eighth attempt. It's the best moment in my life and I want just to keep it. She needed three hours to post an opening-round win over Anastasija Sevastova and another two hours, 12 minutes to dismiss countrywoman Ekaterina Makarova on Tuesday.
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But for the time being everyone at Celtic Park should take heart from the fact the victory would have had Europe sitting up to take notice.
But Saturday's semi-final was a more one-sided affair as Halep broke Ostapenko in the first game of the match.
While Rafael Nadal, Alexander Zverev, Nick Kyrgios and Grigor Dimitrov headline the men's final four in Beijing, the women actually have the bigger event there - $1.271M to the women's victor vs $652K to the men's! Halep will be making her fourth straight appearance in the Republic.
Halep goes through to the quarter-finals where she will face Kasatkina, who started slowly but grew in confidence to see off Radwanska 4-6 7-5 6-2.
Wuhan Open champion Caroline Garcia of France overcame Belgian Elise Mertens 7-6 (4) 6-4, while Ukrainian Elina Svitolina beat Australia's Ashleigh Barty 6-4 6-2 in the second round to set up a clash with Russia's Elena Vesnina.