Maria Sharapova seems to be inching back towards her best form after reaching the Italian Open semi-finals on Friday. Photo: AP Photo/Francisco Seco)

ROME - Maria Sharapova fired a French Open warning with an epic three-hour win over Roland Garros champion Jelena Ostapenko to reach the Italian Open semi-finals on Friday.

Former world number one and five-time Grand Slam winner Sharapova found herself in a dogfight with her Latvian opponent.

Playing her fourth match in as many days, Sharapova needed three hours, 10 minutes to win a gruelling struggle, eventually triumphing 6-7 (6/8), 6-4, 7-5.

For Sharapova, now the world number 40, it was arguably her best victory since her return from a 15-month doping ban last year.

The loss, meanwhile, left Ostapenko with only Rome and Stuttgart quarter-finals to show for her efforts prior to the start of her title defence in Paris a week from Sunday.

Sharapova rode a roller-coaster in her marathon win, working her way from 4-1 down in the first set only to drop it in a tiebreaker after 80 minutes.

She regained her momentum to win the second set but it was tough in the third when she let a 4-2 lead get away.

With Sharapova serving for victory, Ostapenko secured a break for 4-5 thanks to a ninth Sharapova double-fault which yielded a break point.

The sixth seed then saved two Sharapova match points to level 5-5 before the Russian secured victory two games later with a final break in a match marred by 21 cumulative double-faults and a dozen breaks of serve.

In other results, Estonian Anett Kontaveit  upset second seed Caroline Wozniacki of Denmark 6-3, 6-1 to move into the semi-finals  of the Italian Open on Friday.

Wozniacki's defeat came in her first clay meeting with Kontaveit, ranked 26th.

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